Darwin is the core of Mac OS X. The Darwin kernel is based on FreeBSD and Mach 3.0 technologies and provides protected memory and pre-emptive multitasking. Darwin runs on PowerPC and Intel based Macintosh computers (currently there is a difference in the Intel versions of the Darwin and Mac OS X kernels). Some notes on Darwin can be found here.
There is a version of NetBSD for PowerMacs and OpenFirmware PowerPCs, no G5's as of this time.
OpenBSD/macppc runs on New World PowerMacs (most models in the post-beige era).