There's big news in Star Wars fandom: it seems the first-draft script of The Empire Strikes Back has been circulating online for over a month now. (Another timely Mojo news item!)
A history lesson is in order: Shortly after the colossal success of the original Star Wars, George Lucas hired noted pulp SF novel author and Hollywood screenwriter Leigh Brackett to write the script for Empire. She duly turned in a first draft, but sadly died of cancer before she could make any revisions.
Without a co-author, Lucas wrote the next couple of drafts himself, before handing his drafts over to Lawrence Kasdan (the man who also wrote Raiders of the Lost Ark and would go on to script Return of the Jedi) for a polish. But it seems that during the period after Brackett's death, Lucas made some major changes to the story of Empire.
Brackett's script would have created an entirely different series from the Star Wars saga we know today. Most notably, the draft envisions a world where Darth Vader and Anakin Skywalker are two separate people: Luke meets his father's Force ghost while training as a Jedi knight, and then goes on to duel Vader in Cloud City.
Also, Luke has a twin sister who is referred to--but it's not Leia. Instead a girl named "Nellith" is said to have been separated from Luke at birth. Although spoken of, she is never actually seen in the film, setting up a plot thread for future sequels.
(I guess old George really was making it up as he went along.)
The script is available here. Do note that the site is experiencing heavy traffic at the moment, and is periodically shutting itself down to recuperate. So it may be a while before you can access the download page. Nonetheless, you'll likely want to download it before the Lucas Legal lawyers pounce.
EDIT: Mojo now has its own mirror of the script, just in case.
Source: Cinematical