Firefly
Firefly was a space-themed Western show that didn't make it past one season. However, fans were really dedicated to the ensemble cast and their stories. Creators decided to tie up loose ends in a film named Serenity in 2005 and then finally call it quits.
Saturday Night Live
So many classic SNL skits have gotten spin-off films. Michael Meyers brought Wayne's World to the big screen, but we can't forget that John Belushi did it first with The Blues Brothers. Though there have been several films, those are by far the most memorable.