i second the BSD's.... FreeBSD in particular... but that's limited to PC and Alpha architecture.
you might also want to check out linux.
www.linux.org (it'll help you streamline your distribution choices)
however, the two above are "UNIX-like" not really a true UNIX.
for that you need to look at the commercial packages. i dunno if solaris still offers it's Solaris 8 binaries for free. i know it offers the 9 for free, although only on the Sparc platform.