Word for someone who always corrects misspellings

I don't believe there's an exact word for "someone who corrects others' spelling errors", but there is one for a person who is meticulous in spelling, generally:

orthographer (lit. "right writer"): One versed in orthography; one who spells words correctly, according to approved usage.

If there is a single word which indicates (as @ermanen puts it) a "spelling Nazi", it will almost certainly be derived from "orthography" or "orthographer"; you might consider deriving one yourself, or popularizing a new sense for an existing word, such as:

• "orthographizer", derived from "orthographize" meaning "to write or spell correctly", which (I think) nicely emphasizes the transitive nature of his compulsion (i.e. not only ensuring his spelling is perfect, but yours, too), or

• "orthographist", has the advantage of possessing that dogmatic little tail, -ism, but is currently only used to describe a specialist in orthography, or "one who studies orthography". So you can either popularize a new sense for it, or extend it a bit to underscore the adherence to received protocols (an "orthodoxic orthographist"? an "observant othographer"? the possibilities are endless).

EDIT: You might also consider unpopular :)

Perhaps spellchecker? (Whether we like it or not.) [Oxford Dictionary Online]

[Who says computing terms can't be applied to people, as in Thank you, Mr. Spellchecker. Maybe I meant to write check instead of cheque.]

And as @DanBron points out, our slightly stuffier crowd might deem him or her autocorrector.

This is one behavior commonly associated with a pedant is, per Merriam-Webster:

ped·ant noun \ˈpe-dənt\ : a person who annoys other people by correcting small errors and giving too much attention to minor details

There is also spelling nazi as a neologism which is derived from grammar nazi.

Urbandictionary and tvtropes have entries for spelling nazi and there are some usages in Google Books.

• a person who freaks out when a little spelling mistake has occured or has be a constant little a**hole about it.

• people that care more about the spelling of words and correcting them then what the words mean...

[urbandictionary]

Note: There isn't a single word that conveys this idea. Spelling nazi would be the most common colloquial phrase and it is self-explanatory.

^{Source: http://www.documentingreality.com}

Note: "Grammar nazi" is a misnomer that covers all language mistakes which includes spelling. So that might be why "spelling nazi" is derived. Some sources mention as a subtype of grammar nazi.