People have already said most of mine (the usual you're vs. your, it's vs. its, too/two/to, their/they're/there, etc.). But nobody mentioned "sense" and "since". I've seen people write that something "doesn't make since", but that doesn't make sense. People should stop writing that, since it doesn't make sense.
There is also "anymore" vs. "any more". E.g. "I don't watch TV anymore" vs. "I don't want any more grapes."
And "whole entire".