This instructable will show you how to make a simple and cheap atmosphere background lighting system for your PC!
This simple system can be used with the AtmoWin software, or using the AtmoLight plugin in VLC (media player) , it connects to your PC using the standard USB connection on your arduino.
It has only one channel, but it still provides a nice effect.
A video of it in action:
This could be expanded to two channels, for more info see the last step.