It was confirmed by Line 6's engineers that their Chemical X model in Pod Farm is a modded Marshall JMP they got access to.
Also, Waves GTR 3.5 has a couple of different JMP sims, and they actually sound pretty decent.
Amplitube has one (Vintage Metal amp model), but it's ancient and was never particularly good (which is why no one talks about it).
Studio Devil's Amp Modeler Pro has two JMP models (one stock, one with "mid-scooped mod).
That's about I know of.