Actually, while the Electric Company definitely had Spider-Man on there (he never really said anything though- just saw thought bubbles on the screen like in a comic book, with a narrator telling the story) - I know for sure on this one- I've got an episode on tape with Spidey on it!!!
But I've also heard that the Blue Beetle also showed up on The Electric Company. Though he was technically originally a Fawcett Comics character, by the time of The Electric Company, he had been bought by DC Comics, so TEC had both Marvel and DC characters on there. Though I think Spider-Man is remembered a lot more than Blue Beetle.
And I definitely remember the Jughead (from Archie comics) clip on Sesame Street where he told us a lot of "J" words- and the last one was "jump" and he wound up jumping in the river!
I'm not sure if I remember The Pink Panther one or not, but it sounds right. It seems like he probably would have been on there, as well as the Superman and Batman and Robin clips I've heard of. I'm sure there's probably others that have shown up too. Anyone remember some others?
For example, The Simpsons had a cameo on Sesame Street at the end of the original version of "Monster in the Mirror".