Korg Z1 - great for physical modelling
Elektron Monomachine - very many possibilities
Yamaha EX5/7 can do some well weird stuff with the FDSP though it's less knobby than the previous two.

Going off the wall a minute, how about the newer Korg Wavedrum? It's percussion based, has loads of interesting algorithms and is great for exploring, not a great UI though but sound wise may be perfect