Make a nice portable power pack with this 4 x AA battery holder. It fits any alkaline or rechargeable AA batteries in series. There's a snap on cover and an on/off switch which can be handy when wiring to something without a switch.

The four batteries are held in series, for a nominal output of 6V DC for alkaline (6.4V when fresh, 4V when dead), and 4.8V DC for rechargeables (5.2V when fully charged, 4.4V when discharged). Using rechargeables will make this work nicely with nearly any 5V project, with alkalines you may want to put a 1N4001 in series to drop the voltage from 6V down to 5.3V.