As far as 64 bit Unix-type OS' go, yeh, there's a bunch. Linux has been doing 64 bits for years, on SPARC, MIPS, Power, Itanium, and Alpha CPUs. As far as Athlon64 and Opteron specifically stand, I think all the distros are working on it, but it's currently alpha or beta quality. As well as RedHat and Mandrake, at least Gentoo, SuSE, and Debian are working on it.
I read a <A HREF="http://www.osnews.com/story.php?news_id=4618" target="_new">review</A> of Gentoo on Opteron recently and it wasn't too glowing... I'd expect most of the problems to be sorted by the time any formal non-testing releases are made.
I just <A HREF="http://newsforge.com/article.pl?sid=03/09/26/1518235" target="_new">read</A> that SuSE will be releasing SuSE Linux 9.0 Professional for AMD64 next month, which will support Athlon64, and the other distros have beta (test) versions currently available. Give it a few more weeks for bugs to get ironed out, and you should be good to go.
BTW, the RedHat brand is moving fully to the professional market (eg pay for support contract), and the free/home version is to be <A HREF="http://fedora.redhat.com/" target="_new">Fedora</A>, so that's something to keep in mind when the time comes to go looking for it.
<i>Knock Knock, Neo</i>