This is a new Arduino-compatible microcontroller from SFE. The Pro Micro is similar to the Pro Mini except with an ATmega32U4 on board. The USB transceiver inside the 32U4 allows us to add USB connectivity on-board and do away with bulky external USB interface.
This tiny little board does all of the neat-o Arduino tricks that you're familiar with: 4 channels of 10-bit ADC, 5 PWM pins, 12 DIOs as well as hardware serial connections Rx and Tx. Running at 16MHz and 5V, this board will remind you a lot of your other favourite Arduino-compatible boards but this little guy can go just about anywhere. There is a voltage regulator on board so it can accept voltage up to 12VDC. If you're supplying unregulated power to the board, be sure to connect to the "RAW" pin on not VCC.
This board measures in at an amazing 33mm x 18mm! And has a maximum height of 3.75mm (at the micro USB socket).
Note: This latest revision corrects the silk error from the last version of the board so that pin 14 is correctly labeled. We've also added a PTC fuse and diode protection to the power circuit and corrected the RX and TX LED circuit..
Note: This board uses what's known as the "diskloader" bootloader, this is presumably the same bootloader used by the Arduino Leonardo. This bootloader is still in its infancy so expect some glitchiness here and there. Also, because it works over HID you will need to install some drivers so that your computer will recognize it. A "readme" file in the driver pack below explains how to do this.