In 2001, NASA started a pilot program called Clickworkers to allow the general public to participate in their research. Through the program, you can analyze data returned from NASA's Mars missions, such as the HiRISE camera on the Mars Reconnaissance Orbiter, to help NASA identify landmarks and surface details on Mars.
You can also analyze data from Mars Global Surveyor's Mars Orbital Camera (MOC) to help identify the landmarks that NASA should be pointing HiRISE at for further study.
If Mars isn't your cup of tea, they also have data returned from the Dawn mission that you can analyze to mark crater impacts on asteroids (starting with the NEAR asteroid). Doing so will help NASA determine some of the history of the asteroid and our solar system.
With the recent cuts to NASA's science budget, they're going to need all the help they can get, and this is a perfect opportunity to help keep NASA science going in these tough times.
Get started by visiting the Clickworkers website.