Macintosh

Linux can be run on the Macintosh to varying degrees, depending on the processor architecture.

m68k-based

 * The Linux/Mac68k Project
 * Debian m68k port

PPC

 * Yellow Dog Linux
 * Fedora Project (use the PPC build)
 * Ubuntu PPC port (community-supported only)
 * Debian PPC port

Intel x86 and x86-64
Any x86 distro should work to some extent when using Boot Camp. Most newer distributions also natively support EFI.

Distros with native EFI bootloaders (note that EFI support on the Mac may be broken at this time):


 * Ubuntu
 * Debian
 * Fedora

Other links of interest

 * Low end Mac