Linux also runs on several other platforms in varying states of usability:
Alpha (Linux/AXP): http://www.azstarnet.com/~axplinux/
PowerPC processors (LinuxPPC): http://www.linuxppc.org/ and http://linuxppc.cs.nmt.edu/; for PowerUP cards, see the section called Can I run Linux on my Phase5 PowerUP card's PowerPC CPU? in Chapter 8.
PowerMacs using OSF Mach microkernel (MkLinux): http://www.mklinux.apple.com/
Sparc processors (SparcLinux): http://www.geog.ubc.ca/sparclinux.html
I think I heard some rumors about an Intel version too ;-)
This list is not exhaustive. A definitive guide can be found at http://www.ctv.es/USERS/xose/linux/linux_ports.html.