Coolest Vocab Test Ever, yanked from jnanacandra. It's a neat concept: gives pairs of words, and asks you to pick same or opposite for them. While they aren't always exact synonyms/antonyms, if you know what both mean, the choice is clear. There will be many places where you only know one of them, unless your vocabulary is really impressive.
(166/200 for me, just like jnanacandra, though I'd be frightened if it were the same 166.)