Miguel tells us of pash. I was interested in this back when it was called Monad Shell, but never really got into it. The good thing about pash though is that it’s relatively cross platform (Linux, MacOSX, Windows and even WindowsCE). Might have to have another look at it.
Update: Apparently pash is still at the stage where it’s an open source re-implementation of Monad Shell and not quite a Power Shell replacement… yet.