Arduino Micro without Headers

The Arduino Micro is a microcontroller board based on the ATmega32u4, developed in conjunction with Adafruit. It has 20 digital input/output pins (of which 7 can be used as PWM outputs and 12 as analog inputs), a 16 MHz crystal oscillator, a micro USB connection, an ICSP header and a reset button. It contains everything needed to support the microcontroller; simply connect it to a computer with a micro USB cable to get started. It has a form factor that enables it to be easily placed on a breadboard.

The Micro is similar to the Arduino Leonardo in that the ATmega32u4 has built-in USB communication, eliminating the need for a secondary processor. This allows the Micro to appear to a connected computer as a mouse and keyboard, in addition to a virtual (CDC) serial / COM port. It also has other implications for the behavior of the board.
Technical specifications
Microcontroller ATmega32u4
Operating Voltage 5V
Input Voltage (recommended) 7-12V
Input Voltage (limit) 6-20V
Digital I/O Pins 20
PWM Digital I/O Pins 7
Analog Input Pins 12
DC Current per I/O Pin 40
DC Current for 3.3V Pin 50
Flash Memory 32 kB
Flash Memory for Bootloader 4 kB
SRAM 2.5 kB
EEPROM 1 kB
Clock Speed 16 MHz
Lenght 48 mm
Width 18 mm
Links

Datasheet
getting started page
Size