I have written a Z80 target for QEMU, mainly as a learning exercise. It currently emulates the 48K ZX Spectrum.
HPPA host support has been merged into the QEMU trunk, but needs some mork work.
There is now a SourceForge.net project for HPPA (PA-RISC) target support for QEMU.
Laurent Desnogues and I have produced the beginnings of an MMIX target. Not much yet, but it should at least build!
I've also produced the beginnings of an OpenRISC target (with some bits borrowed from the MMIX target). Again, not much yet, but it should at least build!