It's a really good time to be a Duke Nukem fan, as another version of Duke Nukem Forever was leaked on Christmas day. First, we had the early versions of Duke Nukem Forever leak earlier this year--with the 2001 Restoration Mod announced shortly afterward--and now we have another version of DNF made available. It goes back much further than 2001 too, with this version of the game coming from October and November 1996. This version is even an entirely different genre as it's a sidescroller, which isn't as crazy as it sounds. The first two Duke Nukem games were sidescrollers and were released in the early 90s, so for Duke Nukem Forever to originally start out as a sidescroller makes a lot of sense.
If you choose to download this version of Duke Nukem Forever, you can try out different prototype/developer builds of the game, with October 8th, 21st, 24th, and November 8th, 1996 all available. Given that they're all early versions of a canceled game they're not that playable, but you can run around, shoot, and experiment with a piece of gaming history, which is pretty cool.
The leak was from the same group on 4Chan that leaked the 2001 version of the game, and was then quickly picked up by Archive.org, which hosts a repository of them. The files come with the source code, a patched Dosbox setup and a game editor for it, so you can see how they were even making some of the levels.
According to the wiki page for this game, the sidescrolling Duke Nukem Forever was supposed to draw inspiration from the first two Duke Nukem titles while taking the interactivity and character that helped make Duke Nukem 3D such a hit. Returning weapons from 3D ranged from the Mighty Boot to the Shrink Ray, which definitely would've helped make it feel like a true Duke Nukem title. The version was canceled to do a more traditional follow up of sorts to Duke Nukem 3D, in the form of a 3D shooter, which would take well over a decade to finally see some version release. The work would be sold to Inner Circle Creations, who would use it to help make Alien Rampage.