I found this hilarious video on youtube called Grammar Nazis. Unfortunately, the video won't show on anywhere but youtube so I can only link you to it.
Being an English major, I come across the Grammar Nazi a lot. Sometimes, I think it's a little bit ridiculous how intense some people can be. Of course I support using correct grammar, but there are some really obscure rules and people don't adhere to them in speech (among them...who can really tell me when to use who and when to use whom? I never understood that).
However, I admit to having one not-very-well-known grammar rule that I'm a stickler about. That is the difference between using "good" and "well". It's come to the point where my younger brother mocks me and purposely uses the wrong word just to annoy me.
Are you a Grammar Nazi? Know any? Do you have one grammar rule you follow religiously?