And we can't forget the guy singing Mahna Mahna, Bip Bippadotta.

A few others that have stuck with me for 50 years in addition to the more common Cookie Monster, Big Bird, Grover, Oscar, etc. are:
Marvelous Martha (MMMMMMMM, Martha! cartoon segment)
Wicked Wanda (cartoon segment)
Yip Yips (aliens......cow, book, telephone)

This post was brought to you by the letters Q and U and the number 6.

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I love sanding.