OK, I just spent 3 hours plugging my old XP PC, then when that didn't work out installing XP Virtual machine then playing around with it till I managed to install it (it's a 16 bit proprietary installer; not InstallShield or similar) and then messing with dll files till I got it to run, so I hope you appreciate the effort:

Lords of the Realm II demo is a 6-county map that's otherwise not available in the full game.

Well, if you have ever played the game you should know that Mafia 2 does also take place in the 1950's.

There are quite a few unique demos on the 3DS eShop. I know Yokai Watch and Bravely Default both have demos that feature different content from the main game (usually tutorial-ish side quests suitable for a quick-taste demo) but which you can then use in the full-game when you get it. Pokemon Ruby and Sapphire remakes have these demos too.

I have never played it, but Desperados is also said to provide a separate mission in the demo.

+1 for the time investment.

The demo version of Age of Empires: The Rise of Rome has a three level campaign not present in the final game. I tried opening it with the full game a long time ago, but the game crashed.

The campaign was called First Punic War, and its level in order were Struggle for Sicily, Battle of Mylae and Battle of Tunes. You played as the Carthaginians and your enemies were the Romans.

Aliens Colonial Marines.

Rise of the Triad: The demo levels where called "The HUNT Begins" I believe and are included in your GOG/Steam copies. You just have to select them in the setup.exe and boot up the game.

Tiberian Sun: IIRC I think the demo had GDI missions exclusive to it.

FEAR: IIRC this was an interesting one, more like a remix of existing levels.

Half-Life 2: This had an normal demo but it also had a tech demo called Lost Coast consisting of a cut portion of the main game. They bundle it with HL2 as a bonus.

Starcraft: I remember this having exclusive missions.

We are talking about playable demos. Neither the demo nor the full game of Aliens: Colonial Marines were playable.

I see what you did there. ;)

I know this is late but I wanted to add another one I finally recalled. The demo for One Must Fall is an entirely different game from the final release, One Must Fall 2097.

While not a PC game, I'll always remember how the demo for Xenogears varied so much from its final release.

It was just so... different. The demo was way more lighthearted than the actual game itself and included dialogue that broke the 4th wall. What is more, the game didn't even attempt to follow the main story and instead focused on demonstrating the battle system -- allowing access to several characters that weren't available to play until a ways into the game.

Brutal Legend

Not only was it different, it was better than the full game.

Didn't the Descent demo have a level that wasn't in the full game?

Yes, this :

http://www.descent2.com/support/saturn/

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

I actually still have that game in the box, along with the CDROM version of Corridor 7, which I agree, is a somewhat better game than O:BC.