6. Night Skies
Consider, for a minute, that - adjusted for inflation - both Close Encounters of the Third Kind and E.T. are two of the highest-grossing pictures of all-time. That in mind, consider the fact that these movies were both directed by Steven Spielberg, and are both - in very different ways - about aliens. Finally, consider just how much money a third alien-related picture could have made back in the day.
Night Skies, then, was a movie that Steven Spielberg had in development for a while - one which was ultimately shelved and then later cancelled, having been broken up into little pieces and scattered amongst a number of other Spielberg-related projects (mostly E.T., but also Poltergeist).
The coolest thing about Night Skies, however, was that it was intended as a horror film. Whereas Close Encounters was rendered as something akin to a sci-fi drama, and E.T. a family flick, Night Skies would have been Spielberg trying to scare the hell out of everyone - in space! Had it been made, no doubt it would have ended up as something akin to an alien Jaws - with a box office gross to match.