If you've been reading the Hotdog toppings thread, you saw this coming. I've seen cheese, ketchup, and jalapenos. Ironmom says a filling "would be cool", which leads me to think she has never seen them. I can assure you/her they're right in the supermarket!
I think I have also seen mayo in them somewhere, but that might have been Canada. Seeing as putting mayo on hotdogs was never big in the States in the first place. Funny I haven't seen mustard, which would seem more logical than ketchup. Perhaps the market for these is mainly kids. And Canadians.
What else is out there?