I have a Cobalt Flux dance pad laying around, I don't think they're rare exactly but they were quite expensive and since the company went under they're highly sought after, being one of the best dance pads you can get (for the small community that still cares about that stuff)
I don't think anything I own could be considered rare. I have 2 boxed copies of Pokemon Sapphire, one of them CIB, I guess CIB GBA games are fairly rare, since they just came in cardboard that most people threw away. But they still sold lots of them.