When Arduino debuted, most sensors and other interfacing devices ran on 5 volts. Nowadays Xbee radios, accelerometers and other new devices run on 3.3 volts. Here’s a short tutorial from Adafruit showing how to hack your 5 volt Arduino board to run on 3.3 volts. It’s a simple procedure involving replacing the voltage regulator and fuse, and if you can handle soldering small components and need 3.3 volt compatibility, give it a try.
Via Adafruit.