The wii is currently using the gamecube ports for the dancepad. I don't know of any that work for the gamecube and the PC. The red octane pads are pretty good, but not compatible with the wii. Hopefully someone might know of one; for now I am just waiting. Red Octane ,so far, does not seem to interested in wii dance pads. I want an adapter for my afterburner pad, but they don't have one.
Good luck!
For the PC (or xbox, PS2), the afterburner is a great pad if you determine your family will use it enough to justify the cost. After playing on this pad, I won't even mess with a soft pad anymore; it is a completely different experience.