Looking for a simple remote control solution, I came across these small 4x4 button keypads seen in the pictures below. The first two are similar in that they are simple discrete digital circuits with a 8-pin (4x4) output. The only difference is the direction of the pins. The third uses an analogue signal to determine which button is pressed, thus only a single free analogue pin is needed on the Arudino.

(Images link to the Deal Extreme product pages)

Gerhard Schmidt has posted a nice article on how to read these keypads (his examples are with 3x4 keypads, but the principles are of course the same). For the digital 8-pin versions, reading is very simple, and you can even get away with no extra components, if you are willing to spend 8 IO ports on your micro controller. For the analogue based board, voltage thresholds are used to distinguish which key is pressed. Those boundaries will of course depend on the implementation of the board. Deal Extreme links to one circuit diagram, and on Waveshare's product page (they seem to be the original manufacturer) an overview of voltage ranges for both 5V and 3.3V can be found.