For those of you who aren't familiar, folding at home is a distributed computing project that uses the unused processor cycles of millions of computers around the world to perform calculations for the Folding project at Stanford University. The folding project is attempting to understand how proteins fold during the creation of new cells to discover how diseases like cancer and AIDS start and are spread. The end goal is to eventually find the cause, and develop a cure.
It is a very worth while cause and let's be honest, most people have their computers on all the time anyway, might as well let it contribute as it sits there bored all day.
You can read all about the project as well as download the client from here:
After you run it, it downloads some work from the folding at home servers, then, for however long you keep the client installed, it will continue to utilized the unused processor power of your computer to complete work units (WU's). The client only uses idle cycles, so it shouldn't make your computer act any slower, and it uses a very small amount of bandwidth.
The folding at home client also runs on the PS3 (Life with Playstation 3), and you can even join one of many teams that compete for the highest number of credits. For example, here is a link to my folding at home stats.
http://fah-web.stanford.edu/cgi-bin/main.py?qtype=userpage&username=hessmo
As you can see from the stats, I have more than one computer running (my desktop and ps3), but it's not uncommon for one user to have 50 computers acting as a "folding farm". You can see that I am also a member of the Overclock.net folding team.