Pro Trinket is
a ATmega328 based board by Adafruit. It's
compatible with Arduino IDE and most of the code written for Arduino.
It's small factor, cheap and comes with neat accessories like this
LiPo backpack addon.
Pro Trinket is cheaper, smaller and consumes less power than a typical
Arduino board because it lacks things like the
FTDI chip used to communicate
between the programmer and the board.
To program the Pro Trinket you need to have an external FTDI board or
figure out how to make programming over USB work with Linux. I opted
for the first option ...