I first stumbled across Mystery Science Theater 3000 one rainy Saturday. While channel surfing, I landed on Sci-Fi, and saw three sillouetes mocking a cheap rip-off of gremlins. After that, I was hooked.
Mystery Science Theater 3000 has one of the simplest concepts in history. A mild-mannered guy gets shot into space by a pair of mad scientists, who want to get their hands on the world's worst movie. Said guy then builds a few robots to help him watch the horrible movies. The cast performs a few short skits before, after, and in between watching the various pieces of horrid cinema.
The show mainly consists of throwing "riffs" at B-movies. They would point out obvious mistakes, but the real fun was when they would bust out the obscure references, quoting everything from movies and tv shows to ancient philosophy all within five minutes of each other. If a joke flew over your head, another would be right around the bend to remedy it.
That being said, the show isn't for everyone. The humor mostly involves making fun of other people's efforts, so if that humor sounds disrespectful, don't even bother. Production values are incredibly low, although this often adds a nice charm to the proceedings. Finally, a lot of the movies are so awful, they are difficult to sit through even with the riffing. If you can sit through "Monster A-Go-Go" in one sitting, you are a stronger person than me.
That being said, this was one of the simplest, and yet most brilliant, shows ever conceived, and deserves a watching by anyone.