A Grammar Nazi is someone who does their best to enforce and uphold the laws of grammar and spelling. This is sometimes used as an insult, EG, "what are u sum knid of grammer nazi!??!", but it is a term that has been adopted with pride by many.
Many members of the Canon Protection Initiative consider themselves Grammar Nazis, but tend to exhibit different values of Grammar Nazism.
The Department of Technical Errors is full of Grammar Nazis.