Folding @ Home is a Stanford University project of understating protein folding. It is distributed computing where small tasks are sent to Personal Computers (PC) across the world and it uses those CPU power to calculate. The results are sent back to the main computer. It is a very safe program and probably you will not notice any performance change for your computer.
There are similar projects for understanding Genome to AIDS which uses more or less similar technology of distributed computing. If you think you can contribute using your unused power of your PC other than warming the planet than please join any project of your choice.
The recent announcement from Sun about their low power consuming CPU is a new start from those Mega Hertz and Giga Hertz race. As most of the commonly used applications can not use any way as they are always backward compatible to reach all the market.