It's blue! It's thin (0.8mm)! It's the Arduino Pro Mini! SparkFun's minimal design approach to Arduino. This is a 3.3V Arduino running the 8MHz bootloader. Arduino Pro Mini does not come with connectors populated so that you can solder in any connector or wire with any orientation you need. We recommend first time Arduino users start with the Uno R3. It's a great board that will get you up and running quickly. The Arduino Pro series is meant for users that understand the limitations of system voltage (3.3V), lack of connectors, and USB off board.

We really wanted to minimize the cost of an Arduino. In order to accomplish this we used all SMD components, made it two layer, etc. This board connects directly to the FTDI Basic Breakout board and supports auto-reset. The Arduino Pro Mini also works with the FTDI cable but the FTDI cable does not bring out the DTR pin so the auto-reset feature will not work.

The latest and greatest version of this board breaks out the ADC6 and ADC7 pins as well as adds footprints for optional I2C pull-up resistors! We also took the opportunity to slap it with the OSHW logo.

Features:
ATmega328 running at 8MHz with external resonator (0.5% tolerance)
Low-voltage board needs no interfacing circuitry to popular 3.3V devices and modules (GPS, accelerometers, sensors, etc)
USB connection off board
Weighs less than 2 grams!
Supports auto-reset
3.3V regulator
Max 150mA output
Over current protected
Reverse polarity protected
DC input 3.3V up to 12V
On board Power and Status LEDs
Analog Pins: 8
Digital I/Os: 14
Dimensions: 18x33mm

https://www.sparkfun.com/tutorials/244