This website has a lot of media/entertainment type things as well... but there are a host of "games" for geography, literature, science (yay science!), history, language, and sports. (I've spent a lot of time trying to get my best time of naming all 50 states.)

I'm still a Twitter skeptic, but I'm trying to be open minded and come to terms. This was one of the first resources I found to see what other teachers out there are doing with Twitter to make it a worthwhile experience for their students.