The Arduino Protoshield is a Shield board that sits on top of a standard Arduino board.  More information.

This design has a few good features:

• power busses for +5V and GND in middle of board
• central bus can be configured to carry +5V, GND, neither, both (jumpers)
• Special horizontal 5-pin bus to install pin headers for external connections.
• 2x5 IDC connector area for external connections.
• holes connected in groups of five, breadboard-style
• all regular Arduino pins accessible on top
• bottom of board cannot short to USB connector as in other protoshield designs
• power LED and reset button on top
• USB/External power jumper accessible with protoshield installed
• low parts count: only one capacitor, a resistor, and LED and a switch, in addition to header pins.
• two ways to use it: add header pins and a breadboard on top, or just use through holes to wire point to point.