Not exactly a climate experiment but a climate modeling experiment that needs some spare time on our computers. If 10,000 people sign up it will be in effect a computer several times more powerful than the current best on the planet, which is clocking in at about a quarter Petaflop. Here is the link, you can donate spare time on your comp with a screensaver that will be a small chunk of the modeling program.
Here is the link:
http://www.bbc.co.uk/sn/hottopics/climatechange/aboutexperiment1.shtml