Make a portable power brick with plenty of juice!
Use Alkaline AA's for a 9V 3000-4000mAh power supply, or rechargeable NiMH for 2000mAh 7.5V supply. Either one is good for running electronics that have a 5V voltage regulator (thus requiring a 7V+ supply). It will last about 10 times longer than a 9V battery!
Perfect for portable Arduinos! Batteries not included