Cool that you listed PrimeGrid. PrimeGrid is part of a larger network (BOINC - the Berkeley Open Infrastructure for Network Computing). The BOINC network has dozens of projects like primegrid and is the world's largest volunteer computing network. Projects include medical research, the Large Hadron Collider, and a database of asteroids among other things. Recently a project started to crack the genome of the dodo bird.

If you have some spare CPU or GPU power, it's a great way to contribute to something meaningful and get the most out of your hardware. I've been using BOINC for years, it's lots of fun, and essentially free heat in winter if you otherwise pay for electric heat. It's an open-source, permissionless network. Anybody can start a BOINC project and you choose which projects you want to crunch data for.