Blade Runner is the king of the alternate version. Officially there's seven versions of the film, although for the sake of simplicity there's only really three that are essential; the 1982 theatrical release (of which the other cuts are variations of), the 1992 Director's Cut and the 2007 Final Cut.
Because the film was very different to what contemporary audiences were used to from sci-fi - an advancement of the likes of 2001 and the early-seventies reflective ilk, rather than being in the same vein as the more visceral Star Wars and Alien - test audiences did not like it much at all, so a very kneeling version was released, with an added voice-over and tacked on happy drive ending.
Despite these poor elements, the film became a cult hit, to the point where Ridley Scott was eventually able to do away with the studio-mandated changes and, across 10th and 25th Anniversary releases, finally got his original vision realised.
Which Version Is Better? No argument here. The voiceover is dry and the various happy endings are all ridiculous, while the unicorn dream is movie-expanding. The Final Cut is the pinnacle of the sci-fi genre, so it was worth the wait.
Which other movies were released unfinished? Share any more down in the comments.