Appleseed is an open source renderer. You can can export from Blender to Appleseed files using Blenderseed.
Appleseed describes itself as:
appleseed is an open source renderer with a focus on quality and correctness. It started as a platform for rendering research but the end goal has always been to develop a high-end renderer for a wider audience.
The renderer has a good set of features, and I'd like to hear how you think it stacks up against other Blender renderers!