It used to air on the Sci-Fi channel. I just recently started marathoning it on Youtube due to the hype I have for the most recent Rifftrax Live event.
For those who don't know about MST3K, I'll just quote the Wikipedia description, since it's fairly accurate.
"The series features a man and his robot sidekicks who are trapped on a space station by an evil scientist and forced to watch a selection of bad movies, often (but not limited to) science fiction B-movies. To stay sane, the man and his robots provide a running commentary on each film, making fun of its flaws and wisecracking (or "riffing") their way through each reel in the style of a movie-theater peanut gallery. Each film is presented with a superimposition of the man and robots' silhouettes along the bottom of the screen."
It's really fun even though the episodes are pretty long for a TV show. I'd also have to say that I enjoy the pacing much better on the later seasons.