appleseed is a modern, physically-based open source final frame renderer. Started in 2009, it has grown into a robust production rendering toolset and has been battle-tested on several projects such as TV documentaries, ads, promotional videos and animation shorts.
appleseed is available under the MIT license.
More information at http://appleseedhq.net/.
Check out our detailed build instructions to build appleseed from source.
More developer documentation can be found in the wiki.