Taken from the wiki.
NReality is a fan program made by Unreality, downloadable at his website. His description of it is shown below, and more information can be found on its thread in the forums.
It has an additional 100 episodes added to the game. It has added two new modes for the game: speedrun mode & underclock (slowdown) mode. It blocks the direct usage of speedhack. It supports online level highscore recovery (by ctrl-clicking your online score).
Unreality has added new features since it was released. The most recent version has these updates:
It allows for the use of foreground and background images in maps, such as the one in the image demo map.
It can change variables associated with the in-game objects, such as gravity, the ninja's size, and the zap drones' speed.
It also has the ability to save demos played in userlevels or the normal game where the ninja dies.
Recently, NReality has had a huge amount of updates in the area of Modifications, including things such as rotating laser drones, force field drones, wall and ceiling guards, ghost drones, tile drones and many more.
List of Nreality mods.
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I believe it's also been tweaked to be more memory efficient, it certainly runs smoother for me on slower machines.
Standard NED loaded and user levels are fully backwards compatible with the original N1.4.