Reducing Arduino Power Consumption
Contributors:
Alex the Giant
Alex the Giant 
Resources and Going Further
More information on low-power modes and options can be found in the datasheet for the 328P.
For more power-related fun, check out the following SparkFun tutorials.
FemtoBuck Constant Current LED Driver Hookup Guide v13
The FemtoBuck board is a high-efficiency, single-channel constant-current LED driver.
Battery Babysitter Hookup Guide
An introduction and getting started guide for the Battery Babysitter - a flexible LiPo battery charger and monitor.
LuMini 8x8 Matrix Hookup Guide
The LuMini 8x8 Matrix (APA102-2020) are the highest resolution LED matrix available.
TPL5110 Nano Power Timer Hookup Guide
The TPL5110 Nano Power Timer is ideal for applications that require low power, and especially those projects that are running off of a LiPo battery. The Nano Power Timer will turn on your project after the set amount of time, continuously.