A recent implementation for various Z80 based machines is Vezza (or its embedded version) – see also the forum threads (1, 2). Older Z80 ones include M4ZVM and M3ZVM (TRS-80) and ZXZVM (Amstrad/Spectrum).
It looks like the 6502 world is covered by Ozmoo.
Here’s an IFWiki query for the Z-code interpreters for 8-bit systems known to IFWiki (which might not be all of them).