These kind of development boards not only help the engineers, but also help the students and electronics enthusiasts to develop innovative staffs at a very low cost. About 12 years back I bought a 8951 board which was 10-15 times larger than these boards and I paid ~$100. I used that a lot learning and doing several home automation staffs. The enthusiasts now can do much more with these tiny things without spending more money.
@Caleb: Someone brought the Jeenode to my attention...
The frustrating thing for me is that I would have killed to have this sort of thing when I was younger, but there was nothing like these boards available -- and now that they are available (and affordable) I simply don't have the time to play with them (sad face).
Much of the limit to size is sensibly getting useful signals in and out.
None of these boards have gone to die-bond and globbing.
Using standard parts (not bare) I laid out a board a few years ago with a TI uBGA MSP430 DSP on one side, a DFN EEPROM on the other, several OPAs and a bunch of related discrete, thermistors and a pressure sensor, all on 19mm x 6.9mm.
Not a development board, of course, but not pushing the shrink limits all that hard, either. (wildlife telemetry fwiw).
@calebkraft I made it for the DIY Gadget Freak Clinic at DESIGN West 2014. It was fantastic - six of us gave short, five minute presentations of our gadgets and had a lot of fun interacting with the audience. We're doing it again in 2014, if you're ready to get motivated and do something with your FLora!!
I used one to emulate keyboard and mouse movement in a game controller for a kid who has muscular dystrophy. I chose the Teensy 2 because it natively supports HID. I can plug it into any computer and it will be seen as a keyboard and mouse. I don't have to install any software at all.
This makes it a great choice for gaming controllers and I can make it, then ship it to someone. All they have to do is plug it in and it works!