SETI@Home - gotta find those aliens. Been searching for 16 years, one of these days I'll find 'em...or, they'll find me...

I also run WCG - got to do something good with my computing power. At times, I'll let PrimeGrid loose on my computer. And I run WUProp (although it's not whitelisted anymore).

But, my all-time favorite BOINC project back in the day was Pirates@Home. Now, that was a community that knew how to have fun!