This switching regulator lets you power circuits with higher and lower than the voltage of your power source!
Got a device that you want to run for a long time, especially from battery? The Pololu S8V3A Step-Up/Step-Down Voltage Regulator lets you power it from a changing power supply, even if the voltage drops lower than what you need!
This switching regulator takes an input voltage between 1.5V to 12V, and changes it to whatever you select between 2V and 12V - use a single AA battery to get 12V! The only limit is this particular regulator can (at most) supply 300mA to your circuit, but that's definitely a useful amount of current.
The S8V3A regulator has a shutdown feature that can also be used as an under-voltage protection mechanism for batteries that respond poorly to being over-discharged.