Also, people that use "to" when it should be "too".
Or is it "people who use..."?
The most complicated for me was affect and effect:
You can effect an affect, and affect an effect.
Don't even get me started on things like "irregardless", "flusterated", and my new personal fave-to-hate, "it doesn't make a lot of common sense". 
Jincks
The one that bugs me the most is people who use "then" wrong. Dammit, anyway!