Mac-On-Mac
It is a port of Mac-on-Linux for Mac OS X. It is intended to be user-friendly.
Mac-on-Mac: A port of the Mac on Linux project to Mac OS X
With this software, Mac OS X 10.0 through 10.3, Mac OS 9, OpenDarwin, and Linux can be run in a separate window on a Macintosh computer running Mac OS X. The software requires X11 and a PPC-based Mac to run.