George is right, the only usable BeOS's around atm are:
BeOS R5 Personal Edition - the 'home' version of the last version of the BeOS to come out of Be Inc. It is available as a free (and legal) download from
Bebits, however, it is x86-only.
Yellowtab's Zeta (BeOS R6) - not quite finished yet, but a release candidate has just been released, with an upgrade offer for when the final version is finished, x86-only too.
BeOS Exp/Dano (R5.1) - leaked developer version, legal status doubtful, x86-only.
BeOS R5 Pro Edition - same as PE, but with some additional stuff, x86-only ... however, on the cd there's also the R4.5 for PPC, which, to my knowledge is the last version for PPC, but as George said, only for the earlier CPU's, no G3's, G4's.
This version can still be found (with a bit of luck) for purchase, don't ask me where, though.
+ some other distros that for the most part consist of R5 PE bundled with patches and apps, that are also available on Bebits.
So, if your Mac has a 60x series CPU, your best (and only) bet is to find R5 Pro ED for sale somewhere, but be prepared for a lack of apps compiled for PPC.
(Someone please correct me if I'm wrong, I'm in no way an expert on the BeOS)