minix, it was on one of the fisk disks. I never used it because it is very limited, eg i don't think it had a hard drive controller driver...
check it out.. just went to google and searched "minix and amiga" and got:
http://www.cs.vu.nl/pub/minix/
http://www.cs.vu.nl/pub/minix/amiga/
hope that helps