I had some success with Bochs. Afaik it can emulate any of the
Intel CPU families.
It is not without bugs, in particulat the BIOS is limited, and
the harddisk driver doesn't work well (use a boot floppy).
And: beware of vnd(4) and/or msdosfs bugs in NetBSD if you want
to use it the obvious way to create boot images. It doesn't work
over NFS, and it doesn't always update the image file even if you
unmount and unconfigure (must re-configure once more).

Anyway, it is better than nothing...

best regards
Matthias