Trekkie Monster is a character in the Broadway musical Avenue Q who lives in a second floor apartment in a building on Avenue Q, which is a run-down street situated "somewhere way out in Brooklyn or Queens" in New York City. His physical appearance looks similar to that of the Cookie Monster from Sesame Street as well as the Muppet character Sweetums.
Trekkie Monster is obsessed with looking at and collecting porn on the internet just like Cookie Monster is obsessive about cookies. He sings most of the song 'The Internet is for Porn', which has since become a popular Internet meme, especially for World of Warcraft machinima. Trekkie Monster is quite obviously named after the general term for Star Trek followers.
When the London production of Avenue Q opened, mini hand-puppet versions of Trekkie Monster were sold for two days only. As such, these puppets are now highly valued among collectors.