Here's one that bugs me. There, their and they're. They're different words and aren't interchangeble!

Also its and it's. I have to think twice before I type that one.

And, since we're all sharing our pet peeves, here's some that I'm terribly guilty of...(yes, I just ended my sentence with a preposition.)

Affect and effect. What is the proper way to use this? "How will this affect me?"