See the works of Luis von Ahn (http://www.cs.cmu.edu/~biglou/), specifically his GWAP project (http://www.gwap.com/gwap/). He has created several web games that provide training for machine learning algorithms while at the same time entertaining users and brining them back for more. The games are very successful, and from the talk of his I heard, it sounds like the data is proving useful.