Cycles Render Engine
The code is in trunk, in the folder intern/cycles.
Building CUDA binaries on Windows and dependencies from source on Linux require some extra building steps.
There is a bf-cycles mailing list for on topic code questions, design and development discussion. This mailing list is not for feature requests or help using the renderer.
The bf-python mailing list will be used for discussing Render API design and other external rendering integration topics.