The Pro Micro is similar to the Arduino 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.
The (presently unreleased) Arduino Leonardo leverages the power of the ATmega32U4 for all it's functions, including USB. SparkFun has dipped their toes in the water by releasing the Arduino-compatible "Micro" device for those who want to dive into creating USB devices!
This tiny little board does all of the neat-o Arduino tricks that you are familiar with: 4 channels of 10-bit ADC, 5 PWM pins, 12 digital I/O, and hardware serial connections Rx and Tx. Running at 16MHz and 5V, this board will remind you a lot of your other favorite Arduino-compatible boards, but this little guy can go just about anywhere.
Note: The Micro 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. 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.