I have MMX emulator for PC's that doesn't have MMX. (mine have ) It works quite slow but it's good for programs testing.
But it works only under windows (really, it's easy to write your own for DOS)
If you will find any 3dNow! or Pentium II/III emulators, i'll be happy to get a copy
FlyFire/CodeX
http://codexorg.webjump.com