The Machinedrum is a brilliant, special drum machine that can sort of sound like a 808/909, but it isn't as close as the Novation clone, of course.
I suppose that was one of my irks since I love the 808/909 sounds but they are a bit difficult to coax from the machine.
Having said that though, the Machinedrum can make more drum sounds over a larger sonic palette than any of the mention so far. It's a wild crazy box that can do many a special thing.
The best way to understand what I'm saying is to listen to the demo mp3s and hear for yourself what its possible of:
http://www.machinedrum.com/mdsound.php
Keep in mind there are multiple pages, and one of the mp3s is the 12 minute long demosong from the machine.