In this instructable I will teach you how you can run your arduino uno r3 (others will most likely work but are untested by me) using only 2 AAA batteries.
I got everything from Jaycar but I am sure you can source these in countries outside of Australia.
Step 1: Required Item
For this you will need:
1 x Arduino Uno R3
1 x 2 AAA battery holder
1 x Freetronics USB Booster
A Sharp Blade
A USB Cord To Connect Arduino To USB
Step 2: Setup the Boost Board
To use the Freetronics USB Booster with 2xAAA batteries you must cut the track on the 3.75v and join the 1.17v jumper with some solder.
Step 3: Connect the Batteries
Solder the batteries to the input holes in the board and fit the batteries.
Step 4: Connect Arduino
Now, using the USB cord, connect the Arduino Uno R3 and the USB Boost Converter together and you are done.
Make sure the blue PWR LED is on and the red LOW BATT light is off.