For anyone on PowerPC wanting to try a nice OS based on Linux, I second Debian (upstream of Ubuntu) it is quite stable on PowerPC and comes with a variety of DEs easily and has exceptional hardware compatibility in most cases.
Second I would recommend either Gentoo or FreeBSD for people wanting something a little more flexible or DIY. Both are pretty comprehensive, source based Unices (Former is based on Linux with a ports collection similar to the BSDs, latter is the most popular BSD fork currently around) and easy to get around, since they are primarily source based, they also encounter significantly less issues as long as the processor is supported.
