This page can tell you how to make your Target Wii gift card flash along to music via a Winamp plugin, plus command line control.
This is a pretty worthless project, but there's always somebody out there who wants to do it, so have fun.
I basically opened up the gift card as per this page and located the two resistors that you can short to keep the LEDs on. I then put a transistor (honestly can't remember what type or anything) in it with the collector and emitter on each resistor, then connected the middle pin to pin 2 on a PC parallel port. Then I connected another wire from the parallel port's ground (pins 18-25) and the ground on one of the batteries.
Setting data pin 1 high (outputting 1 to port 0x378) would turn the LEDs off, and setting it low (outputting 0 to port 0x378) would turn the LEDs on. That's about it. I'm too lazy to take it back apart for pictures, so sadly, you'll have to live off this.
Here is the software for command line on/off control of the card plus a Winamp plugin. More instructions and the source to both are in the zip.
You're free to do whatever you want with this knowledge...makes no matter to me whatsoever.