I started doing this with SETI@Home. And have continued to run these sorts of
programs on my computers ever since. SETI@Home used BOINC, which is still used by other projects. I also use World Community Grid. Highly recommend!!
I currently have tasks qued/being worked on, from World Community Grid, yoyo@home, and Rosetta@home.
I just started running tasks for yoyo[@home, and by golly. These tasks are freaking HUGE! The shortest task estimate is 2 days, and 18 hours.
Rosetta@home offered up tasks that took me more than a day for most, to be completed.
World Community Grid has been the best in completing tasks and not taking more than a day to finish. The longest estimated time to completion has been under 9 hours.