Matthew Love shares an article on American Theatre that does a great job of briefly describing long-form improv's roots at Chicago's iO (formerly ImprovOlympic), and talks about some of the troupes and performers doing long-form today.
One of the most notable is T.J. Jagodowski and Dave Pasquesi, known as, well, T.J. & Dave. Take a gander below at the trailer for the documentary that chronicles how they bring about their two-man weekly improvised long-form show.
See a scene from one of their shows: