Superman was first connected with "Sesame Street" via an animated insert produced by Filmation (the same company who created the 1966 "The New Adventures of Superman" cartoons), for the first Test Pilot of the children's program. Re-using footage from the cartoon episode "The Chimp Who Made it Big", the "Sesame Street" segment sees Superman up against a giant ape who has Kryptonite vision. Superman uses a lead door to shield himself from the deadly green rays, before briefly announcing the letter "D" for door. It is believed the voice used in this segment was that of Clayton "Bud" Collyer, who also voiced Superman in the 1966 cartoons and in the 1940s radio series.
On January 21, 1971, Superman appeared in a more susbtantial segment on "Sesame Street" (episode 184), where the Filmation animation depicted the Man of Steel teaching children about the letter "S". You can download this segment right here at the Superman Homepage as a 2.6mb MP4 file.
And we can't forget Super Grover, a long-running spoof of Superman. His secret identity is Grover Kent.
After his horse riding accident Christopher Reeve appeared on "Sesame Street" twice. Once on his own, where he recites the alphabet with Ernie and his rubber duckie, and another time with his son Will, where Chris talks to Zoe and Big Bird about being in a wheel chair.