If you encountered a labful or officeful of Macs in the early 2000s, chances are good that a bunch of them were running SETI@Home, the 'contributed computing' project to search through radioastronomy signals for the telltale signs of an extraterrestrial civilization. While the classic SETI@Home application was closed down in December of 2005, the successor client for grid science is alive and well: BOINC, the Berkeley Open Infrastructure for Network Computing, recently updated to version 5.8.15 and happily Universal Binary. OS X users are full peer clients along with Windows and Linux machines.
You say finding LGM isn't your cup of MIPS? You can contribute to plenty of other projects affecting life here on Earth via the BOINC client and the World Community Grid, a 'meta-project' that aggregates work on several key initiatives (protein folding, cancer, climate and AIDS research) and lets you split up your processing power between your choices. You can sign up and start helping immediately; if you like, join the TUAW team and have your contributions tracked with fellow Macnatics. Note that the BOINC client from boinc.berkeley.edu is several versions newer than the one you get from WCG (5.8.15 vs. 5.4.9), so best to download from the source and then register.
Of course, in the interest of environmental sensibility: please don't leave your machine powered on just to run BOINC; save a watt and let it go to sleep when it's truly idle.