Sporacle has a wide array of timed quizzes. Theses are great for classroom competitions as well a individuals trying to beat their own personal best.
The subjects include: science, geography, history, literature, language and others that are just for fun.
I think students could even work in groups on many of these.
I really liked this game where you had to come up with the most common 100 English words in 12 minutes.
Members can also create up to 7 games a month. Membership is free. You do not need to be a member to play the games.
Educational Idea: Students can create games to help them study or to be part of a class presentation.