This is one of the first hardware mods I have seen for the Wii. These guys have created a little module that does different effects with the blue LEDs on the Wii Disc Drive such as playing along with the music. Maybe this video explains it better:
From their website: SICKmods
How can I install a PulseVU into my Wii?
We will be selling manufactured units at this site by the end of the weekend, for just $20. If you're a programmer, you can instead build your own with the soon to be released source code and schematics.