That's a pretty neat trick, fitting 37 I/Os into a 28-pin DIP, two pins of which are used for power and GND. Speaking of which, I wonder how much ground bounce you get if you switch multiple outputs simultaneously?
I think maybe we need to write a guide to all of these so people can see what is out there and compare them. The MBed is a nice piece of hardware, I've seen it used many times. There's also the teensy, digispark, femtoduino, etc.