Hey, I didn't know about this yet: an emulator for various CPUs with just-in-time compilation and support for a whole mix of target and host CPUs. For example, emulating an Intel chip on PPC. Or conversely a PPC on Intel. Or ARM on PPC. And Sparc as a target is already in the works.
Particularly interesting for Linux users: it can do user emulation or system emulation. The latter does what Virtual PC does - present a virtual machine. The former simply offers the ability to run binaries for a different CPU on your own computer, even if you have a different CPU. For example, running Intel binaries on a Linux PPC - without major system emulation.
Due to the just-in-time compilation, the whole thing should also be significantly faster than Bochs. For OS X there's a graphical launcher that also handles the installation of qemu right away. Unfortunately only from OS X 10.3 onwards. Here's the original article.