The Raspberry Pi is a great little cheap computer, but it doesn't have a lot of interfacing options to connect external peripherals. If you want a low-footprint solution that looks great sitting on the Raspberry Pi itself, DIYer Joe Walnes outlines how to make the "Pi Crust" breakout board.
Walnes' design is pretty simple and sits inside the surface area of the Raspberry Pi itself. It adds a wide variety of I/O options provided you're willing to do a little soldering. You order a custom PCB (which Walnes provides the layout file for), and a few other parts amounting to less than $US20.
Once you've got it running, the Pi Crust works with any low-level peripheral, which means you can expand your DIY electronics projects beyond the standard I/O of the Raspberry Pi. Head over to the Pi Crust main page for a list of parts and directions.