Trellis is an open source backlight keypad driver system. It is easy to use, works with any 3mm LEDs and eight tiles can be tiled together on a shared I2C bus. Thus you can individually control each LED behind all 16 buttons. Furthermore you can link two or more boards together for even more inputs.
Trellis is a great way to add a huge amount of input options to your Arduino (or other development board) or evern commands back to a PC via USB and your Arduino. Be sure to order a PCB, elastomer keypad and some 3mm LEDs from the list below. Or if you'd like Trellis on a larger scale - check out the UNTZtrument system.