There's getting to be a number of these 'games with a purpose'
Jane McGonigal is a leader in this area- this is her game:
http://worldwithoutoil.org/Zooniverse is a collection of science projects where scientists ask the public's help in analyzing data.
They're all fun, and all very serious, important scientific projects
http://www.zooniverse.org/There's also a number of projects that require vast amounts of computing power. Some of these projects have asked people to run a program on their computer that does a tiny fraction of the total work in the background when their computer is idle.
You can help search for extraterrestrial life here
http://setiathome.ssl.berkeley.edu/and you can help with many other great art and science projects through BOINC
http://boinc.berkeley.edu/