The guys over at Flipmu have created the Arduinome. The Arduinome is an open source version of the Monome you can build yourself with off-the-shelf parts. The final result costs a fraction of what an original Monome would cost, with the added bonus that you can incorporate extra features into the design (and, of course, when you show it off you can say you made it!).
The Monome
If you don't know what a Monome is, it's a array of illuminated buttons that performs no function until used with special software. The Monome is primarily used in musical situations, but can be programmed to do whatever is needed. The most common uses of the Monome are as a step sequencer and/or a performance tool, but many apps have been created for it to do many weird and wacky things.
The Arduinome
The Arduinome is a beautiful realisation of the Monome, using SparkFun button pads. You can use 3-colour LEDs in the project for excellent feedback options. The Flipmu guys have provided everything you need to create your own, including instructions, parts list and code.
The only thing is that it doesn't include a case. You can buy bezels from SparkFun, but most people opt to built their own front panel (or a few guys with laser cutters out there are happy to do it too). How you mount it is up to you. For instance this guy mounted his in a chopping board and uses it as a step sequencer...