This shield, available from Sparkfun, gives you the bare minimum for a protoboard - header pins, a soldering area, a reset pin, and a status LED. It also comes with two pushbutton switches that you can use as you wish in your prototyping. Arduino sells a similar model here.
This protoshield is almost identical to the first, except that it expands out to give you a bigger area for components. The header pins are extended to screw terminals at the side so there's more room in the middle and you can screw in even more components to the edges. Adafruit also has a version, available here.
If you're trying to stack two shields on top of eachother but they won't fit or keep interfering, stick this shield in between to fix your problem. If your shields are competing for the same pin, you can re-route signals by re-wiring this shield to move pins around. Handy for more ambitious projects.
If you're trying to develop a shield but you're not quite sure it will work yet, use this shield to interface between your Arduino and your work in progress. You can screw on the board in development without soldering anything, allowing you to experiment without making any commmitments.