Game controller development board
This post also represents contributions from Kirk Lau.
The following is a response to a question from Josef about why the Arduino Pro Micro is the best choice.
The core issue in using the pro mini is that - much like other boards in the $10 range - it lacks usb communication. In order to communicate with the board, as we really need to do to have a usb controller, we need either a usb-to-serial converter or a microcontroller capable of directly using the protocol. Although these $10 boards can afford the processor chip, LEDs, resistors, and such, the usb-to-serial chips cost about $15 alone. (Note that the Adafruit Arduino Pro Trinket and Arduino Fio have mini-usb ports, but these are only capable of uploading new sketches and powering the board, respectively. It's not a two-way connection).
This leaves us with a few options for usb communication. We can buy $20 FTDI cables, which have the usb-to-serial chip embedded inside. However, Kirk and I both used these in E11 and they're unreliable, bulky, and fragile. We can buy $15 FTDI boards, but that's still fairly pricey, adds another board to deal with, and is still bulky and a point of failure. We could use a board that has the FTDI chip and the microcontroller both onboard, like the Arduino Nano, but these are a good $30 and difficult to find (not very popular).
Our best bet is using a board equipped with the ATmega32u4 chip. These were released less than two years ago, and have a built-in usb transceiver, eliminating the need for any serial communication whatsoever. There's a $23 Adafruit Pro Micro that uses this, as well as a $20 Sparkfun Pro Micro.
Yes, the pro micro is twice the price of the pro-mini, but when you take into account the extra hardware we'd have to buy to allow communication to the PC with other boards, it's actually the cheapest option available. If you want six analog inputs, the Adafruit pro Micro has plenty. If we only need four and want to allow students to buy a cheap ADC chip if they really need more analog signals, we can save a couple bucks with the sparkfun version.