The Freeduino SB-Protoshield offers handy 4-switch / 4-LED / Servo / XBee prototyping space for your Arduino / Freeduino SB.
If you want to do anything relatively custom to an Arduino-style microcontroller, you'll find a board like this immensely handy. You can even stick a mini-breadboard onto the top of the soldering area!
Note: There are male header pins included with this kit so it is not natively stackable, the female pins included are for breadboard and breakout purposes. If you'd like some stackable headers we have those too, just grab 2 of the FPIN8L-413 and 2 of the FPIN6L-413.