Look up manufacturer based on Mac Address?

I want to incorporate into a piece of software the ability to look up a manufacturer based on a mac address. By googling "mac address lookup" and similar, I have noticed several websites that make this correlation which suggests this data source is available somewhere. Where can I find this data source that correlates a mac address (input) with a manufacturer (output)?

The first half (24 bits) of your mac-address is called an OUI (Organizationally Unique Identifier), and identifies the company. The list is available on ieee.org:

http://standards.ieee.org/develop/regauth/oui/oui.txt

They are formatted like this:

00-03-93   (hex)        Apple Computer, Inc.
000393     (base 16)    Apple Computer, Inc.
                        20650 Valley Green Dr.
                        Cupertino CA 95014
                        UNITED STATES

The gaps between sequential hex-numbers are probably Privately Registered OUI's. There is no open list for those, but I've never encountered a MAC-address in such ranges.

As @Bart De Vos says the IEEE maintains the canonical list of OUIs. Additionally there is an Individual Address Block (IAB) list at http://standards.ieee.org/develop/regauth/iab/iab.txt. However, entries can be privately registered which means that they will be omitted from oui.txt and iab.txt. The Wireshark development team maintains a much more complete list:

https://code.wireshark.org/review/gitweb?p=wireshark.git;a=blob_plain;f=manuf

We pull entries from the IEEE's public OUI and IAB lists, individual contributors and any other source we can.

The Nmap team maintains a separate list at http://nmap.org/svn/nmap-mac-prefixes.