While I've always known about SETI, it simply never really caught my interest.
But this blog entry by Onno brought to my attention some other projects that might interest you.
By downloading and installing a small program on our computers, we can make use of our computer's idle processing power. Those of us who leave our computers turned on day and night are wasting alot of computer power that could be put to good use, for the benefit of science and mankind.
Visit Berkeley's site to see a list of projects that you can participate in.
For the technically challenged; by installing this program, as long as you're connected to the internet and the computer is idle (not doing anything demanding), the program will run scientific calculations using your computer and then send the results of those calculations back to Berkeley for collation. You can help further scientific research by donating those spare cpu cycles!!
Or maybe you just want to help find aliens by joining the SETI project ;)
Admit it, leaving your computer on 24/7 to download torrents isn't really that demanding on your cpu, why not help out a good cause?