Commonly Confused Words
Sound-alike and look-alike pairs with different meanings: affect/effect, their/there/they're, your/you're, to/too/two, accept/except, then/than, lose/loose.
Most pairs are homophones, which spell-checkers cannot catch. Other regulars: principal/principle, complement/compliment, stationary/stationery, capital/capitol, and advice (noun) versus advise (verb).