Sporcle
sporcle.com
A huge library of user-made trivia quizzes and games, many of them timed, on every topic from geography and history to pop culture.
Why it is useful
The recall-style format, where you type as many answers as you can before the clock runs out, is a good test of what you actually know. With so many quizzes, there is almost always one on a topic you care about.