Boinc, also known as the Berkeley Open Infrastructure for Network Programing is a open source project started by Berkeley. You download the software, then you sign up for the projects you wish to participate in. After this is done, Boinc will sit by passively and wait until your computer is idle. When the computer is no longer being used for anything, Boinc will take over, and download and perform calculations for the projects you selected to participate in, and upload the results. As soon as you start using the computer again, Boinc will return to it's passive state, so that you won't even notice that it is running. This way your computer can make itself useful in the name of science, whenever you aren't using the computer, but still leave it running.
The BOINC network consists of up to 540.000 computers and servers from all over the world, and has a combined computing power of 832 000 000 000 000 calculations per second. Needless to say, this is an invaluable resource for scientist worldwide.
You can sign up for all kinds of projects in many different areas, like astronomy, medicine, climate research, math and so on, for instance:
You can select how much of your computers processing power you want Boinc to have, how much memory and how much of your harddrive it may use. you also can select how long you want the computer to be inactive before Boinc should start.
You can also go the the Boinc homepage and se how much you have helped your project. You get points for the capacity of your processor multiplied with the time your Boinc has used on the project. This way you can compare yourself with other users. You can register in groups, for instance you can get together with friends, or your company, and compete with others.