A person who's (seemingly) sole objective online is to correct a persons spelling/grammar. They perform the same task as spellcheck but in a more hostile/annoying manner.
Person 1: "i like cheese"
Person 2:"The correct way to type it would be "I like cheese." dipshit."
Person 1: "omfg ur such a spelling natzi! skule ees four da lousers nurd!!!11one"