From the technologizer blog is the story of how Jim Henson made quite a few short films with some familiar faces before he went on to make Sesame Street and The Muppet Show. They were made between 1966 and 1976, and include such classics as this:
(Note the teeth: a feature dropped later on.)
For more videos and more on this story, check out the blog.
(Video: Cookie Monster for IBM – YouTube)