Your or You're...that is indeed the question

This post will be a running list of grammatical pet peeves of mine. now, I'm under no illusions that I'm an English PhD professor, but there are some nonsensical blunders that irk me so very badly.

• Definately ( if you can't tell what's wrong with that word then ... Shoo)
• "Could of" instead of Could have. This makes my skin crawl...