If you want a full distribution, also check out Yellow Dog Linux.

Their <a href="http://www.yellowdoglinux.com/resources/ftp_mirrors.shtml">download</a> section has a nice simple list of where to get it, and the "supported machines" list covers iMac as well!

I haven't used Yellow Dog myself, however. Does anyone else use it? Is it a good, full featured Linux distribution?

Also, Neo, could this be added to the FAQ section? I just noticed there is no information for Mac systems...