I've removed incorrect info about
Powerslave demos (which actually contain levels from the full version of the game (maybe with slight, hardly noticeable differences), and added info on beta versions of
Duke Nukem 3D,
Powerslave and
Shadow Warrior.
There's a good site dedicated to the beta analysis of
Duke Nukem 3D and
Shadow Warrior:
http://hendricks266.duke4.net/d3dbeta.html
http://hendricks266.duke4.net/swbeta.html