World's smallest development kit
I've just got one sample ToolStick devolopment kit. It consists of a USB dongle and a credit card sized CD with the software. As the ToolStick is USB powered there is no new for an external power supply. The drawback is that it only contains a couple of outputs connected to a pair of LEDs so you can check whether your software is working as expected or not.
It is certainly a minimalistic setup and the easiest one to carry around. I'm not sure it can be very useful for general purpose development, but it seems like a cheap tool to be used (and given away) in the the classroom. A ToolStick costs $10.99 plus shipping. If you are happy with just two output LEDs I do not think any other system can beat this one as this one contains a nice Windows IDE and C compiler.