It's blue! It's skinny! It's the Arduino Pro! SparkFun's minimal design approach to Arduino. This is a 5V Arduino running the 16MHz bootloader (select 'Arduino Duemilanove w/ 328' within the Arduino software).
Arduino Pro does not come with connectors populated so that you can solder in any connector or wire with any orientation you need.
The DC power jack footprint is available, but not populated. You can run it off a battery, or solder the 2.1mm DC jack to the board and run it from a 9v-12v DC power supply.
We really wanted to minimize the cost of an Arduino. The Arduino Pro is like the Arduino Duemilanove (same pin out) but to keep the cost low, 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 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.