AROS is an opensource reimplemation of AmigaOS(3.1+MUI), but it ain't binary-compatible
(unless you run it on a 68k-CPU). Every SW has to be recompiled in order to run on any
other version (mainly x86), but that should be fairly easy as AROS uses the same API.
For x86 you can get 2 versions:
a) Native : Boots directly from the BIOS, and takes over the whole machine. Problem is that
there aren't that many good drivers, so you may run into trouble if you have the "wrong" HW.
b)Hosted : Runs on top of Linux/x11, and therefore should run on every x86 from 386 to P4.
Also the only way to develop for AROS atm.