'Grey’s Anatomy,' ABC (2005-present)
What started out as a midseason replacement became one of ABC’s top-rated series. Featuring a diverse cast of budding stars and entertainment veterans, the show received critical acclaim in its early seasons. Grey’s has weathered its share of off-screen drama: Isaiah Washington’s homophobic comments and Katherine Heigl’s complaints about working conditions. The show spun-off another medical drama, Private Practice, that gave supporting actress Kate Walsh her own lead role.
Why it’s still on -- and how much time it has left: ABC values one of the shows that made the network competitive again, and it is still relatively strong in the ratings, especially when factoring in DVR playback. The end of surgical residency dovetailed nicely with the stars' contracts. But since the network has resigned most of the cast for two more seasons, it looks like the writers will have to explain why none of the doctors accepted job offers from other hospitals. (However, the last two episodes of Season 8 give them some room to maneuver.)