NetBSD and OpenBSD have had good PowerPC support for quite some time, whereas FreeBSD's port is relatively new. You'll want the 'macppc' port for any Apple system.

Been a while since I've used a PPC, but I'm guessing hardware support under Linux and NetBSD probably wouldn't differ too much these days.

I do remember Linux being slightly less fiddly on PPC when it comes to boot loaders.