Here's another suggestion my brother sent me that is of particular interest to people who enjoy words and expanding their vocabulary.
Artwiculate is a Twitter-based word of the day competition. To play you just use the word of the day in the proper context in one of your tweets. Other tweeters vote as to how effective your use of the word was in your tweet. You can earn points when people like your use or retweet it. But for wordsmiths, the real fun is in constructing a tweet using the word in the best way possible and in seeing how others use the same word of the day.