SparkFun PRT-09497 ATX Power Supply Connector - Straight
This is a 24-pin Molex Mini-Fit Jr.? connector, commonly used as the power supply connector on ATX motherboards. Two rows of twelve pins are broken out two rows of through-hole male headers. See datasheet for the pin spacings and other dimensions.
Pair this connector with a standard computer power supply and you can source a number of different voltages (+3.3V, +5V, +12V, -12V, -5V) to your project.