There's also yellotab zeta. It's not free (pretty cheap though), and I haven't tried it, but would like to one day. It is also physically possible to install MacOSX on recent x86 boxes, though not strictly legal, I think.
Apart from Aros, I'm not sure that anything will be very similar to Amiga OS. Some have said that OS2 was similar. I think it's also out of date now, though.
