
http://www.opensciencegrid.org/
That way you can backfill the resource with jobs from VOs (Virtual Organizations, think projects/groups) you choose. It is similar to BOINC in the sense you can pick what science to support.
The idea is that the sharing will go both ways. You will give spare cycles to other users on OSG, and in return, you will be able to use spare cycles on other resources.
Spend some time on the OSG website reading the things under "Learn About Us". Also check out the research highlights to see what kind of science is being done on OSG resources:
(Shameless plug - I'm part of this team) The good news is that OSG have an ongoing effort to help you join your resource to OSG, and help your users get going on the grid:
Thus spake the master programmer: "When a program is being tested, it is too late to make design changes." -- Geoffrey James, "The Tao of Programming"