The Movie Blog directed me to a wonderful quiz by Empire Online: Can You Guess All These Movies From Just These Letters?
I got 22/46. Not quite half, but enough to feel good about not having a life.
They also have a "How Well Do You Know [Insert Movie Title]" section, but you have to be logged in to play. Still, it's worth it because the questions are good. I got 11/20 for Friends, 15/20 for Breakfast Club, 16/20 for West Wing... sadly, I only got a 10/20 for Die Hard, but those questions were freakin' hard.
And if you're a fan of poster taglines, you can try to guess these taglines.