Google toolbar just got updated to enable the new Google Compute on toolbar.

Google Compute is a new feature of the Google Toolbar. The goal of Google Compute is to make a contribution to science by enabling Google Toolbar users to easily donate their idle computer time to worthwhile causes. The Google Compute version of the Google Toolbar detects when a computer is idle and puts it to work on complex mathematical problem from such fields as drug design and global climate modeling.
The first beneficiary of this effort is Folding@home, a non-profit research project at Stanford University that is trying to understand the structure of proteins so they can develop better treatments for a number of illnesses. In the future Google Compute may allow you to donate your computing time to other worthwhile endeavors, including projects to improve Google and its services.
FAQ's here, Google weblog here