With a simple MIDI shield and some basic circuitry, it's easy to interface an Arduino with the world of digital music. This gives you the ability to not only create your own diverse range of musical instruments - but also for your Arduino to be controlled via the MIDI interface. This introduces the ability to create some interesting music-related lighting effects. One example of this has been published by Instructables user "mross13" whose project can detect each MIDI note and act upon it, for example display a colour with an RGB LED:
With more LEDs and a little more work, you could create a large, music-responsive light show. Nevertheless, visit the project page to get started. And for more, we're on twitter and Google+, so follow us for news and product updates as well.
If you're looking to make your own MIDI-controlled light or other project, consider our Freetronics RGBLED: full colour module. It includes a bright RGB LED on the top of the board and a WS2801 constant-current, addressable, multi-channel LED driver on the back. This smart module can be daisy-chained, so you can connect a number of these together in a string and drive each of the module colours individually from your microcontroller. For more information and to order, visit the product page.