I've been playing around with some Greenarray[1] chips lately, and I have to admit that the core ideas are fascinating. Their particular variant of Forth is actually almost pleasant to use--certainly better than MIPS, which is my closest point of reference. (Ignoring crazy things like the color of identifiers controlling your program's semantics.) The chips are also extremely power efficient.

[1]: http://www.greenarraychips.com/

I've found having a mental model of a stack-based system far easier than even a simple register-based one. However, this might also be true because the Greenarray chips themselves are far simpler than any other CPU I've ever looked at.

And, of course, the fact that you have 144 cores on a tiny chip despite a cheap manufacturing process is also very neat.