NetBSD is an open source BSD derived UNIX Operating System It can trace its roots back all the way back to First Edition Unix from Bell Labs.
NetBSD's goals are to support as many Hardware platforms as possible, They currently support arm32,mac68k (in vairous guises), sparc, sparc64, alpha, amiga, atari, hp300, i386, mvme68k, pc532, pmax, sun3, sun3x, vax, x68k.
They claim to be the first *NIX with USB support too...