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Sparkfun's intros their own iteration of the Arduino board -

Skinny is the first SparkFun hack into the Arduino realm. It's not perfect, but we think you'll like this new derivative. It's 3.3V, runs on LiPo batteries, low-cost, low-profile, and sexy red. Which is actually one of the reasons why it cannot be a certified Arduino board (Arduino boards have to be blue).
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• ATmega168V running at 8MHz internal oscillator
  
• Low-voltage board needs no interfacing circuitry to popular 3.3V devices and modules (GPS, Accelerometers, sensors, etc)
  
• USB connection off board
  
• 3.3V regulator
  
• DC input 3.3V up to 12V
  
• Resettable fuse prevents damage to board in case of short
  
• Power select switch acts as on/off switch