Adafruit Prototyping Pi Plate Kit for Raspberry Pi can snap onto the Pi PCB and gives you all sorts of prototyping goodness to make building on top of the Pi super easy.
There's a big prototyping area, half of which is 'breadboard' style and half of which is 'perfboard' style so you can wire up DIP chips, sensors, and the like. Along the edges of the proto area, all the GPIO/I2C/SPI and power pins are broken out to 0.1" stips so you can easily connect to them. On the edges of the prototyping area, all of the breakout pins are also connected to labeled 3.5mm screw-terminal blocks. This makes it easy to semi-permanently wire in sensors, LEDs, etc. There's also a 4-block terminal block broken out to 0.1" pads for general non-GPIO wiring. Finally, there is a little space remaining over the metal connectors so an SOIC surface mount chip breakout area is added for those chips that don’t come in DIP format
This kit comes as a set of parts, a little soldering is required to put it together but it’s really easy, even a beginner can do it in 15-20 minutes or so.
CAD Files: https://github.com/adafruit/Adafruit-Prototyping-Pi-Plate
Adafruit Website: http://www.adafruit.com/products/801