Taking Open Source concepts and Cloud computing together, vSwarm is promising a free-to-access Render Farm. Currently they target the Blender and LuxRender software.
You can do mainly two things to participate:
- use the Render Farm to process your renderings
- install a Client software to contribute CPU power to the farm (you'll need to install a VMWare-powered client system, which is currently only available for Windows, but Linux and OSX support are planned).
More info on http://www.vswarm.com/