Adafruit Arduino Micro with Headers - 5V 16MHz - (ATmega32u4 - assembled)
As of February 12th, 2016 the Arduino Micro is now being manufactured in the United States in partnership with arduino.cc!
Squee! It's the cutest, tiniest little Arduino yet! The Arduino Micro packs all of the power of the Arduino Leonardo in a 1.9" x 0.7" (48mm x 18mm) size. Although it may look like a stick of gum, its actually a USB-native 8-bit microcontroller, with 32K of flash, and 2.5K of RAM. You can program it directly via the USB micro connector on one end (or for the advanced users, the 6-pin ISP header). The Arduino Micro is a microcontroller board based on the ATmega32u4 (datasheet), developed in conjunction with Arduino.cc and Adafruit.
Onboard is the processor, crystal, micro USB connection with fuse, Reset button, ISP low-level reprogramming header, ON LED, TX and RX LEDs, a extra user pin #13 LED, and a 3.3V regulator. You'll probably also want to pick up a micro-B USB cable to connect this board to your computer
You can use it for basic Arduino sketches as well as USB client projects like making it appear as a mouse or keyboard to your computer! Although it is not shield compatible - it does have all the same pins as a Leonardo. Its best for when you want to shrink your project down or use it on a breadboard.
This version of the Micro comes with headers pre-soldered on and runs at 5V logic with a 16MHz crystal.
For more technical details and starting guides, check out the Arduino website