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Here's Conan O'Brien doing the Monorail Song.
The Stonecutter's Song
Yeardley and Nancy doing Minimum Wage Nanny
They apparently closed with Do the Bartman, and here is Weird Al and Matt Groening (and others!) doing the dance:
Creator of the Waving Universe
Crack!Fic, The Marge Simpson Way: "Just then, Sir Lancelot rode up on a white horse and saved Joan of Arc. They got married and lived in a spaceship. The end." - Tales from the Public Domain, "Hot Child In The City"