OpenDarwin was a collaboration project with Apple in the early and mid-2000s to produce a (primarily) command-line Darwin derivative (that is, an OS based on the Darwin kernel) and to provide an environment in which to build and develop Mac OS X source code. The project unfortunately was shut down in 2007. In the words of the project administrators, it had become "a mere hosting facility for other open-source projects," which was nowhere near its intended goal.
System Requirements: For PPC, same as Mac OS X v10.3.2, i.e. 128 MB RAM, G3 or newer processor; for Intel x86, unknown but probably similar to PPC requirements, i.e. 128 MB RAM
Issues: AirPort wireless cards on PowerPC hardware are detected, but cannot be configured; power management does not work.