Where is PMX available?
As for Linux, most distributions are still using X though they're trying to get away from it. It's hard to make another graphics layer that everyone agrees to and most programs are written for X.
I will note that X, or rather the session managers and window managers are a lot more familiar to people now, as in working more like Windows then traditional X where copy and pasting is done by highlighting something and middle clicking to paste and scroll bars scroll in different directions based on which mouse button you click. Even on OS/2, X has its own clipboard.
As for programs, at one point we had quite a few. QT, GTK etc were all ported. Mozilla first ran on X, just fired up Warpzilla 0.98. About the only page it would load was Hobbes
Sadly most of the old programs seem to be lost, especially the source code.