Anyone know of any game demos with unique content or level(s) not included in the main game? The only one I can think of is RE7 from a few years ago. I'm sure there are more from my youth I can't remember or didn't realize at the time. I did a quick search but only found lists of best game demos. These typically included entries where you could play the first chapter or a couple hours of the game but its essentially still the main game.

Penumbra Tech Demo offers a very different location and unique foes for the initial part of the same story. I enjoyed this demo as much as the game itself. :)

I suppose both Warcraft 3 and Starcraft demos apply to this? Both had prologue missions.

Kinghts of the Chalice

The demo is an entirely separate scenario, and many consider it to actually be harder than the full game.

There are two that spring to mind

Half Life: Uplink
[url=https://half-life.fandom.com/wiki/Half-Life:_Uplink]https://half-life.fandom.com/wiki/Half-Life:_Uplink[/url]

Apparently this was a level cut during development and repurposed as a demo.

Hellpoint: The Thesbian Feast contains content not found in Hellpoint.

Didn't know about Half Life Uplink. Not sure about Hellpoint. It's labeled a demo but the game card says sequel of sorts. Don't know what to make of that.

Tiberian Sun's demo provides GDI missions not present in the full game.

Desperados: Wanted Dead or Alive demo has an unique mission/map that's missing from the base game.

https://www.youtube.com/watch?v=wAvO2KnwiV4

The Space Quest 6 demo.

The Stanley Parable's famous demo.

The Rusty Lake game, The Past Within, has a unique puzzles in its demo, but it's kind of hard to get on PC because GOG doesn't have it and Steam won't let you own a demo and a full game at the same time. Thankfully, it's easy to pick up on Itch. It's also on smartphones, where it's called The Past Within Lite.

Hikage_XjS mentioned the Starcraft demo, though it's worth noting that the levels were fan-ported to the main game and can be easily played without installing a separate version. I bet the Warcraft levels are out there too. Warcraft 1 also had slightly variant demo levels, including a "level" where you automatically lose. You can read about those at TCRF.

Half-Life Uplink was also released as a demo for the PS2 version of the game. In fact, a few days ago, RetroAchievements added an achievement for the thing. Go figure!

Final Fantasy XV's demos were each based on older versions of the game, with the Platinum Demo including a unique story, but good luck trying to get a playable copy today.

I'm pretty sure Bravely Default 1 and World of Final Fantasy had demos with at least slightly distinct content. Even if they didn't, playing their demos did unlock content in the main game, which is another topic worth discussing.

The shareware episode of the original Rise of the Triad was, AFAIK, a completely separate set of levels from those in the full game.

There is a Demo included in Carrion, one of craziest games, and it's somehow different than original. All prologues should be extra chapters.

Jedi Knight II: Jedi Outcast demo consisted of a single, unique level.

Didn't know that. Do you happen to know where this can still be obtained?

Also Desperados III got a demo level that is not in the game, according to the devs.

I think, also Surviving the Humans belongs to this list.

Then of course there are games which had demo versions very early in the development where the games changed drastically over time or where the games were canceled.
System Shock Remake first demo(s), Ion Maiden, Unsung Warriors, IJ and the fountain of Youth, IJ and the Seven Cities and I don't know why I kept the demo to Kokopa's Atlas.

I found this one. I've downloaded it, but didn't try it to see if it works yet.
https://jkhub.org/files/file/2701-jedi-knight-ii-jedi-outcast-demo/