Check out Cosmology at Home. Remember how SETI set up this neat program that was your screensaver, but while it was displaying neat little graphs and junk it was calculating information important to SETI's mission? This is like that, only without the screensaver and much, much cooler. You download the program, sign up for projects that interest you and while you are surfing away on the internet or downloading porn or whatever it is that you do with your computer, it uses your compy to run programs that calculate stuff for protein folding databases and climate analysis.
Then you can say, "Look at me! I'm doing astronomy without even trying!"
On another note, I am riding my bicycle to Brest tomorrow for the weekend. Cross your fingers that I don't get lost.
Correction: The program for Cosmology at Home, BOINC, will run while your computer is not connected to the internet, so this is cool. Also, there is a screensaver feature, but I'm not sure how it works.