As long as you have power left, powering Arduino-based projects with batteries opens up a world of possibilities. As long as you have power left.
There are dozens of Arduino compatible boards, each focusing on a specific niche. Rocket Scream's Mini Ultra 8 MHz is the answer for anyone looking for a low power board. A fully compatible board, the Mini Ultra 8 MHz runs using the minimum amount of power possible and comes with optimized low power libraries to help you eek every last second out of your battery.
If you're planning a battery-powered project, the Mini Ultra 8 MHz is worth a look.
Now on InMojo: Rocket Scream's Mini Ultra 8 MHz
InMojo supports the Open Source Hardware Definition v1.0