On the Intel Macs, you actuall can run all systems natively, from FreeBSD over Linux to Windows and OS X. There are some catches with the boot loaders, but several magazines have already published articles about it.
On the PPC Macs... Well. Ubuntu does a great job on them, and it is good to know that there will be a free operating system available the day that Apple no longer supports OS X on the old machines.
On the PPC Macs... Well. Ubuntu does a great job on them, and it is good to know that there will be a free operating system available the day that Apple no longer supports OS X on the old machines.