F@H uses your excess computer power to fold proteins, which can help scientists understand why certain things happen.
A lengthy explanation is at their website:
http://folding.stanford.edu/English/ScienceIt ONLY uses excess power, i've had it running for two days now and i've yet to notice any drop in performance on my pc. Keep in mind, if you have a really low end pc/laptop you probably don't wanna run this, even on the lowest setting, because the small impact it does have will be more noticeable.
You can chose what disease you wanna help research, so if someone you know has had or does have one of the diseases, this is a great way to help out.
Before someone says this is a virus, hundreds of thousands of people have and still are using this.
It's really easy to install and the download size isn't big.