Luis Von Ahn and Laura Dabbish have an article in the August 2008 CACM on "Designing Games with a Purpose".
The paper has a nice overview of the goal of Games with a Purpose (GWAP), which is to produce useful output from the work done in games, and a good survey of some of the games available at gwap.com and the useful data they output.
If you've seen the GWAP work before, what is new and interesting about the article is the general framework they describe for building these types of games. In particular, the authors describe four generic types of guessing games, give examples of each class of games, and help guide those that are thinking of building their own games. In addition, Luis and Laura give a fair bit of advice on how to make the games enjoyable and challenging, how to prevent cheating, and techniques for mixing and matching human and computer players.
If you haven't seen GWAP before, go over to gwap.com and try a few games. My favorite is Verbosity, and Tag a Tune is good fun. I also think Tag a Tune is impressive as a demonstration of how games that label audio and video can still be quite fun even though they take a lot more time to play.