I used to be a SETI addict, but just have my two slower machines running that now. On my main box I run BOINC, which is a GUI client that runs seperate processes and shares CPU usage. I am currently running a climateprediction.net and the LHC@home projects. I haven't yet converted my SETI account to BOINC.
For more information about BOINC and the computing projects they are involved with check out
http://boinc.berkeley.edu/