Anyone who's been online for a little will probably be aware of Grid computing projects like the SETI@Home. For those that don't know, these are scientific projects which take advantage of the fact that there are so many exceptionally power personal computers in the world with literally masses of spare unused processing power.
Ordinary non-scientific people are able to take part by installing some software and when their machine is idle it does number crunching on specific data. In the case of SETI@Home the user takes radio wave data from a section of outer space and analyses it for unusual noises that could indicate extra-terrestrial intelligence. Other projects have included cracking encryption algorithm and sequencing DNA.
Just to get an idea on how good these projects can be, and why people should take part if they can, there was a project recently called WISDOM (World-wide In Silico Docking On Malaria) which ran from October 1st 2006 to January 31st 2007. During that four months the project was able to complete work that would've taken a single PC 420 years to do.
If anyone is interested in getting involved then might I suggest this cancer research project. If you want to get involved you will need to install something like Boinc (that is the Windows version as I'm assuming most people are running it.