I donate my computer's idle time to help solve complex problems through World Community Grid.
World Community Grid (WCG) takes part in grid computing, where small computations are distributed to many computers to solve a single difficult task of a project. There are many computations and tasks for every project, and rather than just give it to supercomputers to solve, it is given to many computers all over the world to do.
There are many projects that you can be a part of to help make the world a better place. You could be a part of the research to help find the cure for AIDs or cancer by simply installing the program and running it on your computer as a screensaver when you aren't using it, you would be a part of a huge network of individuals all over the world on this initiative to help accelerate research in humanitarian causes, scientific development, and technological breakthroughs.
As it runs as a screensaver, it also looks pretty cool!