Best Linux Distro for PPC

I've been wanting to put my second HD to good use. Its empty, so I think i'll put Linux on it. Don't know much about Linux for the PPC, so what do you guys think is the best distro? The point is to learn Linux better, not for it to be average PC user friendly. Also, what about the whole DualBoot thing? Will it mess up my OSX install or anything, I don't have the OSX discs.

Remember im putting it on a completely separate HD, not on the same one with OS X. Thanks.

http://www.terrasoftsolutions.com/products/ydl/

That's Yellow Dog Linux. It seems to be in active development too which is cool. There is a PPC version of Ubuntu as well, but they didn't include it in their latest release (however, the last release of it was 6.06 LTS and is supported until 2009). You can get that here:

http://releases.ubuntu.com/6.06/