PulseVU for Xbox & Xbox360
The PulseVU, previously available for the Wii, is now available in Xbox & Xbox360 versions with even more features. This device pulses any LEDs you connect to it with the beat of the music coming from your Xbox. In v1.6 Xboxes and in the Xbox360, it also has the ability to fade your LEDs when the system is off. It can easily be turned off without adding a switch and can cycle through its 10 different modes at anytime.
You can find more information and install directions at http://pulsevu.sickmods.net.