Deadline's Robert Lang compiled a bunch of short films (that you can watch for free online) that were later developed into feature-length films like Reservoir Dogs, Marcel the Shell with Shoes On, Boogie Nights, Bottle Rocket, Napoleon Dynamite, and District 9.
For instance, here's Quentin Tarantino's original Reservoir Dogs:
Wes Anderson's original Bottle Rocket:
The original short version of Marcel the Shell with Shoes On:
Peluca, upon which Napoleon Dynamite was based:
The Dirk Diggler Story, the short film by PT Anderson on which Boogie Nights was based:
