I leave my computer on 24/7 because I'm either downloading something or burning something or otherwise working at it. I would leave it on 24/7 even if I didn't have this great little program, but it's one reason to leave it on. It's definately the reason why I watercool my computer to overclock it so far.
Anyways, this is a program that can be downloaded from Stanford (they distribute it and collect the data). Data is released to the public, published, and is not sold in any way. This program helps analyze protein synthesis and folding in attempts to better understand it and perhaps find a cause or cure for alzheimers and other related diseases.
Check it out, this is seriously the best thing you can do for the world while just surfing the internet...http://folding.stanford.edu/
P.S. A note of caution. This will bring your CPU usage up to 100%, so don't use this program if your computer has overheating problems. It does tax the system resources a tad, but for the most part it simply uses unused processing power. I have this installed on every system I own. I actually have my 2.0Ghz Athlon 64 3000+ (yeah, socket 754 and single channel sucks, I know... if I had money I'd build a new system around the Venice core) overclocked to 2.4Ghz (this thing will beat a pentium 4 running at 3.6Ghz).