If you're looking for an easy way to get a regulated 5 V for your electronics project, this battery holder from Bodhilabs is just what you need. It comes with a step-up DC-to-DC converter on the back that can provide up to 200 mA (typical; 500 mA max), and there is a jumper that can be removed to disconnect the batteries from the regulator and give you direct access to the battery voltage. We also sell the regulator PCB separately.
The leads coming off the regulator are terminated by a 0.100" 2-pin female connector into which you can plug standard 0.100" male header pins.
For more information on the voltage regulator, please see the VPack PCB datasheet (95k pdf).