Global warming is a serious issue. You can support the research done in that field by donating some of your underused computing horsepower to climate prediction. That's an Oxford University research project using remote computing on distributed machines all over the world to compute climate models, much like SETI@home, which seems to be moving to the same platform now.
A word of warning though: If you are uncomfortable using the command line (Terminal application), then this is nothing for you yet because the installation unfortunately requires some manual unpacking and changing of permissions, using UNIX commands. I'll make a note when they come up with a Mac user friendly package.