There are some os9 emulators for os x although I haven't heard any stories on how they work on Leopard. I myself haven't upgraded to Leopard yet but i will someday, that's why i'm holding on to an old G3 iMac for os9 related work.
: They're using Tiger. :)
: More helpfully, there's Pfhorge, which is pretty complete but not very
: Forge-like. There's Forgery, which is considerably less complete (doesn't
: even save to the Marathon map format :( ) but has an OS X binary ready.
: Finally, there's Smithy, which is both a Forge clone and complete up to
: VISUAL MODE and a few unimplemented texturing features but has to be
: compiled from source, an arduous task since it's written in OCaml and GTK+
: (which in turn means it has to be run under X11, at best like Gimp.app).
: I don't know if Basilisk II can be made to work on OS X, but that's an
: emulator for pre-OS 9 (I think) machines, and so if you had a Mac ROM
: image and a disk image (or a pre-OS 9 installation disk), you could
: emulate Forge and Anvil. Again, this is assuming that it works at all.
: Personally, if you're man enough, you should attempt Smithy. If you aren't
: man enough, you should attempt Basilisk and, failing that, Pfhorge.