This is a very popular topic, I find.

A things that bug me:

Your - you're
Misspelling of "necessary". (tricky word, to be fair; it took me a while before it sunk in, but now it bugs me)
Definite - definate
There - their - they're
Two - too - to
Double negatives do my head in.
People who rudely and probably obnoxiously go around correcting people.

Probably lots more. But hey, when I was younger, I probably did all those mistakes, and I know for sure I made plenty.

They might be annoying, but it's important not to be an arse about it, as we all make mistakes, and some people are dyslexic.