x86. This is the main port.
64-bit port. There is some preliminary work for a x86_64 port.
ARM port. Is not in a usable state.
PowerPC. Is not in a usable state.
Alpha: project I, and project II. Was once started, but isn't in a usable state either.
MIPS. Status completely unknown.
Mach on Top of POSIX. Status unknown.
When starting a port for a new architecture, it might make sense to first target a paravirtualized environment, that already abstracts away some of the different hardware implementations' quirks.