CorsixTH is an open-source project meant to create a replacement runtime/engine for the classic Bullfrog game Theme Hospital. It's in somewhat active development and in a state of early releases and it does require assets from the original game disc or demo like early OpenTTD releases, but it's already flawlessly playable from start to finish with the exception of certain in-game challenges (i.e. rats and epidemic, bonus levels).
I expect that the rest of the community will create open-source versions of the levels and other required files to make the game independent from the original once the codebase is feature-complete, like OpenTTD did.
The license of the game implies that it's Open Source, but does not appear to use GPL/LGPL/BSD as those are not mentioned in the main portion of the license.
Site of the software: http://code.google.com/p/corsix-th/
https://code.google.com/p/corsix-th/wiki/GettingStarted?tm=6
"3. The first time you start the game it may ask for the location of Theme Hospital. If it does not you are done! Happy gaming!
Otherwise, point CorsixTH to an installation, the CD (the HOSP folder) or somewhere else you have put Theme Hospital. If you don't have Theme Hospital already you can buy it at gog.com or download the demo here. As an example, if you did buy the original game from www.gog.com and installed it on Windows using default settings you will want to point CorsixTH to C:\Program Files\GOG.com\Theme Hospital on first startup."
But maybe it can be done with online installer that will download the demo.
Code license is MIT License. It an open source compatible license.