http://www.worldcommunitygrid.org/
World Community Grid uses the processing power of computers connected to the internet. The processing power is being used for many not-for-profit researches (list available in the website). All you need to do is to create an account, install a small piece of software in your PC/ Mac and its done. Its absolutely safe.
Most of the time, we do not use 100 percent of the CPU and this can be used for such purposes. You do not have to worry about starvation of your processes, as you can set the preferences like the percentage of CPU and disk space that can be used by world community grid and ensure that the processing power of your system is used only when you are not using it.
Teams
There are also the concept of teams within world community grid.
Basically, you and your friends can create teams and contribute as a team. All the points you and your friends gather will be part of the team. Ie You can contribute collectively. This is very good when an organization wants to contribute.
There are also various challenges from time to time where you can associate yourself and contribute towards the challenge as a team. There is also one small restriction which i think is kind of right. That is... You can only be part of one team. If you create a team, you will be the pilot of the team. If you create a new team, you will be removed from the older team automatically.
You also have the privilege of selecting the projects to which you want to contribute.
Now they have also introduced the concept of contributing to Beta projects.
Hope I have given enough introductory information about world community Grid.
for more... check out the website.
I can set one up if anyone wants (but then I'll have to leave BOINC@AUSTRALIA 
)