Depending just how old a Mac you want to keep alive, NetBSD is probably a good option. It's been years, but I ran it on a TiBook and found most of the onboard devices to be reasonably well supported.
You won't get a glossy, high-performance graphical desktop from a default NetBSD install -- you'll have to turn to pkgsrc for that. However, I'd generally warn against running anything but OS X on even an old mac if you want a pretty graphical desktop and decent performance; the OS X GUI is *much* lighter weight than KDE or GNOME...