Today I joined to World Community Grid project by registering on http://www.worldcommunitygrid.org. This community conducts various research projects which are beneficial for humanity.

The projects are
1. Nutritious Rice for the World
2. AfricanClimate@Home
3. Discovering Dengue Drugs - Together
4. Help Cure Muscular Dystrophy
5. Human Proteome Folding - Phase 2

They take a small amount of computing power when your pc is idle and they send the data to the main server which can be utilized by the application to get the required results.
Here you volunteer by giving your idle processing power.You get the satisfaction that you are part of a system which is contributing for making this world a better place to leave.

To get more information on grid computing visit this wiki.There are many project like this you can find it on this site.