A few of my favorites:
Elmo scares Julia Roberts
Bill Cosby recites the alphabet
Patrick Stewart's soliloquy on B
Celebrities sing the Alphabet Song
There are many more. A quick search and you can find Ricky Gervais singing a celebrity lullaby to Elmo (hysterically scary); Little Richard performs a fabulous Rubber Ducky number (move over Ernie); and a whole bunch featuring yip, yip martians meeting the telephone, or a fan, or a clock... The list goes on.