You might also consider Mac OS X and its open source BSD foundation, Darwin. Mac OS X is mostly proprietary, but everything up to its BSD layer is available for free download at http://publicsource.apple.com/ - it can be made to compile on PowerPC and Intel x86, and John Carmack has succeeded in porting X Windows to it.
I've not had a chance to use it myself, but from all accounts it will mostly be of interest to BSD fans who want to experiment.