Where do I find the current C or C++ standard documents?

PDF versions of the standard

As of ~~1st September 2014~~ September 2021, the best locations by price for the official C and C++ standards documents in PDF seem to be:

C++20 – ISO/IEC 14882:2020: 198 CHF (about $217 US) from iso.org
C++17 – ISO/IEC 14882:2017: $90 NZD (about $65 US) from Standards New Zealand
C++14 – ISO/IEC 14882:2014: $90 NZD (about $65 US) from Standards New Zealand
C++11 – ISO/IEC 14882:2011: $60 from ansi.org or $60 from Techstreet
C++03 – INCITS/ISO/IEC 14882:2003: $30 from ansi.org
C++98 – ISO/IEC 14882:1998: $80 NZD (about $60 US) from Standards New Zealand
C17/C18 – INCITS/ISO/IEC 9899:2018: $116 from INCITS/ANSI / N2176 / c17_updated_proposed_fdis.pdf draft from November 2017 (Link broken, see Wayback Machine N2176)
C11 – ISO/IEC 9899:2011: ~~$30~~ $60 from ansi.org / WG14 draft version N1570
C99 – INCITS/ISO/IEC 9899-1999(R2005): $60 from ansi.org / WG14 draft version N1256
C90 – ISO/IEC 9899:1990: $90 NZD (about $65 USD) from Standards New Zealand

Non-PDF electronic versions of the standard

C89 – Draft version in ANSI text format: (https://web.archive.org/web/20161223125339/http://flash-gordon.me.uk/ansi.c.txt)
C89 – Draft version as HTML document: (http://port70.net/~nsz/c/c89/c89-draft.html)
C90 TC1; ISO/IEC 9899 TCOR1, single-page HTML document: (http://www.open-std.org/jtc1/sc22/wg14/www/docs/tc1.htm)
C90 TC2; ISO/IEC 9899 TCOR2, single-page HTML document: (http://www.open-std.org/jtc1/sc22/wg14/www/docs/tc2.htm)

Print versions of the standard

Print copies of the standards are available from national standards bodies and ISO but are very expensive.

If you want a hardcopy of the C90 standard for much less money than above, you may be able to find a cheap used copy of Herb Schildt's book The Annotated ANSI Standard at Amazon, which contains the actual text of the standard (useful) and commentary on the standard (less useful - it contains several dangerous and misleading errors).

The C99 and C++03 standards are available in book form from Wiley and the BSI (British Standards Institute):

C++03 Standard on Amazon
C99 Standard on Amazon

Standards committee draft versions (free)

The working drafts for future standards are often available from the committee websites:

C++ committee website
C committee website

If you want to get drafts from the current or earlier C/C++ standards, there are some available for free on the internet:

For C:

ANSI X3.159-198 (C89): I cannot find a PDF of C89, but it is almost the same as C90. The only major differences are in the boilerplate and section numbering, although there are some slight textual differences
ISO/IEC 9899:1990 (C90): (Almost the same as ANSI X3.159-198 (C89) except for the frontmatter and section numbering. There is at least one textual difference in section 6.5.7 (previously 3.5.7), where "a list" became "a brace-enclosed list". Note that the conversion between ANSI and ISO/IEC Standard is seen inside this document, the document refers to its name as "ANSI/ISO: 9899/99" although this isn't the right name of the later made standard of it, the right name is "ISO/IEC 9899:1990")
TC1 for C90: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n423.pdf
There isn't a PDF link for TC2 on the WG14 website, sadly.
ISO/IEC 9899:1999 (C99 incorporating all three Technical Corrigenda): http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1256.pdf
An earlier version of C99 incorporating only TC1 and TC2: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1124.pdf
Working draft for the original (i.e. pre-corrigenda) C99: http://www.open-std.org/jtc1/sc22/wg14/www/docs/n843.htm (HTML) and http://www.dkuug.dk/JTC1/SC22/WG14/www/docs/n843.pdf (PDF). Note that there were two later working drafts: N869 and N878, but they seem to have been removed from the WG14 website, so this is the latest one available.
List of changes between C89/C90 and C99: http://port70.net/~nsz/c/c89/c9x_changes.html
TC1 for C99 (only the TC, not the standard incorporating it): http://www.open-std.org/jtc1/sc22/wg14/www/docs/9899tc1/n32071.PDF
TC2 for C99 (only the TC, not the standard incorporating it): http://www.open-std.org/jtc1/sc22/wg14/www/docs/9899-1999_cor_2-2004.pdf
ISO/IEC 9899:2011 (C11): http://www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf
ISO/IEC 9899:2011/Cor 1:2012 (C11's only technical corrigendum): This can be viewed at https://www.iso.org/obp/ui/#iso:std:iso-iec:9899:ed-3:v1:cor:1:v1:en but cannot be downloaded. It is the actual corrigendum, not a draft.
ISO/IEC 9899:2018 (C17/C18): https://web.archive.org/web/20181230041359if_/http://www.open-std.org/jtc1/sc22/wg14/www/abq/c17_updated_proposed_fdis.pdf (N2176)
C2x work-in-progress - latest working draft as of 18th October 2020 (N2731): http://www.open-std.org/JTC1/SC22/WG14/www/docs/n2731.pdf

For C++:

ISO/IEC 14882:1998 (C++98): http://www.lirmm.fr/~ducour/Doc-objets/ISO+IEC+14882-1998.pdf
ISO/IEC 14882:2003 (C++03): https://cs.nyu.edu/courses/fall11/CSCI-GA.2110-003/documents/c++2003std.pdf
ISO/IEC 14882:2011 (C++11): http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2012/n3337.pdf
ISO/IEC 14882:2014 (C++14): https://github.com/cplusplus/draft/blob/master/papers/n4140.pdf?raw=true
ISO/IEC 14882:2017 (C++17): http://www.open-std.org/jtc1/sc22/wg21/docs/papers/2017/n4659.pdf
ISO/IEC 14882:2020 (C++20): https://isocpp.org/files/papers/N4860.pdf
ISO/IEC 14882:2023 (C++23 work-in-progress. Working draft dated March 17 2021): http://open-std.org/JTC1/SC22/WG21/docs/papers/2021/n4885.pdf

Note that these documents are not the same as the standard, though the versions just prior to the meetings that decide on a standard are usually very close to what is in the final standard. The FCD (Final Committee Draft) versions are password protected; you need to be on the standards committee to get them.

Even though the draft versions might be very close to the final ratified versions of the standards, some of this post's editors would strongly advise you to get a copy of the actual documents — especially if you're planning on quoting them as references. Of course, starving students should go ahead and use the drafts if strapped for cash.

It appears that, if you are willing and able to wait a few months after ratification of a standard, to search for "INCITS/ISO/IEC" instead of "ISO/IEC" when looking for a standard is the key. By doing so, one of this post's editors was able to find the C11 and C++11 standards at reasonable prices. For example, if you search for "INCITS/ISO/IEC 9899:2011" instead of "ISO/IEC 9899:2011" on webstore.ansi.org you will find the reasonably priced PDF version.

The site https://wg21.link/ provides short-URL links to the C++ current working draft and draft standards, and committee papers:

https://wg21.link/std11 - C++11
https://wg21.link/std14 - C++14
https://wg21.link/std17 - C++17
https://wg21.link/std20 - C++20
https://wg21.link/std - current working draft

The current draft of the standard is maintained as LaTeX sources on Github. These sources can be converted to HTML using cxxdraft-htmlgen. The following sites maintain HTML pages so generated:

Tim Song - Current working draft - C++11 - C++14 - C++17 - C++20
Eelis - Current working draft

Tim Song also maintains generated HTML and PDF versions of the Networking TS and Ranges TS.

POSIX extensions to the C standard

The POSIX standard (IEEE 1003.1) requires a compliant operating system to include a C compiler. This compiler must in turn be compliant with the C standard, and must also support various extensions defined in the "System Interfaces" section of POSIX (such as the off_t data type, the <aio.h> header, the clock_gettime() function and the _POSIX_C_SOURCE macro.)

So if you've tried to look up a particular function, been informed "This function is part of POSIX, not the C standard", and wondered why an operating system standard was mandating compiler features and language extensions... now you know!

POSIX.1-2001: The System Interfaces section can be downloaded as a separate document from https://mirror.math.princeton.edu/pub/oldlinux/download/c951.pdf. Section 1.7 states that the relevant version of the C standard is C99.

The "Shell and Utilities" section (https://mirror.math.princeton.edu/pub/oldlinux/download/c952.pdf) mandates not only that a C99-compliant compiler should exist, but that it should be invokable from the command line under the name "c99". One way in which this can be implemented is to place a shell script called "c99" in /usr/bin, which calls gcc with the -std=c99 option added to the list of command-line parameters, and blocks any competing standards from being specified.

POSIX.1-2001 had two technical corrigenda, one dated 2002 and one dated 2004. I don't think they're incorporated into the documents as linked above. There's an online HTML version incorporating the corrigenda at https://pubs.opengroup.org/onlinepubs/009695399/ - but I should add that I've had some trouble with the search box and so using Google to search the site is probably your best bet.

There is a paywalled link to download the first corrigendum at https://standards.ieee.org/standard/1003_1-2001-Cor1-2002.html.

There is also a paywalled link for the second at https://standards.ieee.org/standard/1003_1-2001-Cor2-2004.html

There is a draft version of POSIX.1-2008 at http://www.open-std.org/jtc1/sc22/open/n4217.pdf.

POSIX.1-2008 also had two technical corrigenda, the latter of the two being dated 2016. There is an online HTML version of the standard incorporating the corrigenda at https://pubs.opengroup.org/onlinepubs/9699919799.2016edition/ - though, again, I have had situations where the site's own search box wasn't good for finding information.
There is an online HTML version of POSIX.1-2017 at https://pubs.opengroup.org/onlinepubs/9699919799/ - though, again, I recommend using Google instead of that site's searchbox. According to the Open Group website "IEEE 1003.1-2017 ... is a revision to the 1003.1-2008 standard to rollup the standard including its two technical corrigenda (as-is)". Linux manpages describe it as "technically identical" to POSIX.1-2008 with Technical Corrigenda 1 and 2 applied. This is therefore not a major revision and does not change the value of the _POSIX_C_SOURCE macro.

Online versions of the standard can be found:

Working Draft, Standard for Programming Language C++

The following all draft versions of the standard:
All the following are freely downloadable
2021-06-18: N4892
2021-03-17: N4885
2020-12-15: N4878
2020-10-18: N4868
2020-04-08: N4861

This is the C++20 Standard:
This version requires Authentication
2017-04-08: N4660

The following all draft versions of the standard:
All the following are freely downloadable
(many of these can be found at this main GitHub link)
2020-01-14: N4849
2019-11-27: N4842
2019-10-08: N4835 git
2019-08-15: N4830 git
2019-06-17: N4820 git
2019-03-15: N4810 git
2019-01-21: N4800 git
2018-11-26: N4791 git
2018-10-08: N4778 git
2018-07-07: N4762 git
2018-05-07: N4750 git
2018-04-02: N4741 git
2018-02-12: N4727 git
2017-11-27: N4713 git
2017-10-16: N4700 git
2017-07-30: N4687 git

This is the old C++17 Standard:
This version requires Authentication
2017-03-21: N4660

The following all draft versions of the standard:
All the following are freely downloadable
2017-03-21: N4659 git
2017-02-06: N4640 git
2016-11-28: N4618 git
2016-07-12: N4606 git
2016-05-30: N4594 git
2016-03-19: N4582 git
2015-11-09: N4567 git
2015-05-22: N4527 git
2015-04-10: N4431 git
2014-11-19: N4296 git

This is the old C++14 standard:
These version requires Authentication
2014-10-07: N4140 git Essentially C++14 with minor errors and typos corrected
2014-09-02: N4141 git Standard C++14
2014-03-02: N3937
2014-03-02: N3936 git

The following all draft versions of the standard:
All the following are freely downloadable
2013-10-13: N3797 git
2013-05-16: N3691
2013-05-15: N3690
2012-11-02: N3485
2012-02-28: N3376
2012-01-16: N3337 git Essentially C++11 with minor errors and typos corrected

This is the old C++11 Standard:
This version requires Authentication
2011-04-05: N3291

The following all draft versions of the standard:
All the following are freely downloadable
2011-02-28: N3242 (differences from N3291 very minor)
2010-11-27: N3225
2010-08-21: N3126
2010-03-29: N3090
2010-02-16: N3035
2009-11-09: N3000
2009-09-25: N2960
2009-06-22: N2914
2009-03-23: N2857
2008-10-04: N2798
2008-08-25: N2723
2008-06-27: N2691
2008-05-19: N2606
2008-03-17: N2588
2008-02-04: N2521
2007-10-22: N2461
2007-08-06: N2369
2007-06-25: N2315
2007-05-07: N2284
2006-11-03: N2134
2006-04-21: N2009
2005-10-19: N1905
2005-04-27: N1804

This is the old C++03 Standard:
All the below versions require Authentication
2004-11-05: N1733
2004-07-16: N1655 Unofficial
2004-02-07: N1577 C++03 (Or Very Close)
2001-09-13: N1316 Draft Expanded Technical Corrigendum
1997-00-00: N1117 Draft Expanded Technical Corrigendum

The following all draft versions of the standard:
All the following are freely downloadable
1996-00-00: N0836 Draft Expanded Technical Corrigendum
1995-00-00: N0785 Working Paper for Draft Proposed International Standard for Information Systems - Programming Language C++

Other Interesting Papers:

2021 / 2020 / 2019 / 2018 / 2017 / 2016 / 2015 / 2014 / 2013 / 2012 / 2011

C99 is available online. Quoted from www.open-std.org:

The lastest publically available version of the standard is the combined C99 + TC1 + TC2 + TC3, WG14 N1256, dated 2007-09-07. This is a WG14 working paper, but it reflects the consolidated standard at the time of issue.

Draft Links:

C++11 (+editorial fixes): N3337 HTML, PDF

C++14 (+editorial fixes): N4140 HTML, PDF

C11 N1570 (text)

C99 N1256

Drafts of the Standard are circulated for comment prior to ratification and publication.

Note that a working draft is not the standard currently in force, and it is not exactly the published standard