Over on Banterist, Brian Sack has a great recurring bit called "Grammar Cop," where he includes photos of publicly displayed grammatical errors. Check out today's, as well as some of his archives.
Meantime, here's my initial list of grammar pet peeves. Feel free to suggest others.
Your and You're
Your means belonging to you. Your pimples are huge.
You're is short for you are. You're going to have to do something about those pimples.
Its and It's
Its means belonging to it. I love that restaurant but I can't remember its name.
It's is short for it is. The restaurant you love? It's Mama Leone's.
Their, There, They're
Their means belonging to them. The twins got their tattoos at that tattoo parlor.
There is a location that is not here. Yeah, that tattoo parlor over there.
They're is short for they are. They're very small tattoos.
Would've, Could've, Should've
Would've, Could've and Should've are short for Would have, Could have and Should have.
Would of, Could of and Should of are wrong, no matter how right they may sound!