You can watch various segments of Christopher Reeve's appearance on "The Muppet Show" via YouTube.
The spin-off internet show "Statler and Waldorf: From the Balcony" (which ran biweekly on Movies.com from June 2005 until September 2006) featured an episode (#27) in which a Muppet Superman tries to return a DVD to the rental store. When he learns that he's going to have to pay a late fee, and the clerk won't believe that he was out saving the world, Superman reverses the rotation of the Earth in order to turn back time (much like the plot element from "Superman: The Movie"). Clark Kent's "cousin", Craig Kent, also appears in this episode with an editorial commentary wherein he laments being overshadowed by his cousin Clark.
There are many, many more Superman references throughout various Muppet TV shows, movies, magazines and merchandise. Check out the Muppet Wiki entry on Superman and DC Comics for more crossover connections between Superman and The Muppets.
Christopher Reeve also has a connection to the Muppets. Visit http://muppet.wikia.com/wiki/Christopher_Reeve for all the details.