(The Title is a famous Calvin & Hobbes line and book title, but apropriate.)
I remember being one of the first to join the Seti@Home project. I installed it on a number of computers at one time, but forget about it over the years. (I also briefly flirted with the Distributed.Net client.) I just recently got reinterested with BOINC (the Berkeley Open Infrastructure for Network Computing). BOINC allows you to pick amongst several great scientific endeavours to donate your computer's idle time to. When you install these you get some of the coolest screensavers that you can find, particularly when you can point at it and say "It's really doing something; this means something!".
The BOINC client is a bit funky (probably because it was designed by college students :-P), but once you get it installed it takes care of itself. One of the big problems for me was that BOINC didn't properly trigger the Windows Firewall "allow this program" dialog. It is real easy here to visit the Firewall Settings, Exceptions and Add Program. On the Installed Programs box BOINC was right near the top.