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, things are still too much in motion for that.
The bf-python mailing list will be used for discussing Render API design and other external rendering integration topics.