Climateprediction.net is a huge computer program designed to produce a forecast of the climate in the 21st century.  You can donate your computer's processing power if you have broadband access.  The system will utilize your resources (while they are not in use by you) to help compute the climate forecast.

http://www.climateprediction.net/

This system makes use of shared processing time by coordinating the computation of seperate code blocks across the Internet.  You might recall that SETI uses a similar architecture to compute their radio signals.

Their first set of results have just been released:

http://www.nature.com/news/2005/050124/full/050124-10.html