OpenTTD is an open source simulation game based upon Transport Tycoon Deluxe

What made OpenTTD a great success that it's playable out of the box. Most open source game remakes [0] are engine-only and you still need the graphics, arts, sound, and music assets to make it a 100%, and actual playable experience. OpenTTD started like that too but from the very beginning it was a goal to "detach" the game from the original as quickly as possible. It was released in 2004, the actual graphics replacement project started in 2007 and by 2009 100% of the sprites were finished so the original game files were not needed anymore [1]

And actually there are now 5 different basesets on top of the original TTD one [2]

It also made possible things like releasing the game on Steam and GOG.

0, https://github.com/radek-sprta/awesome-game-remakes

1, https://wiki.openttd.org/en/Archive/Community/Graphics%20Rep...

2, https://bananas.openttd.org/package/base-graphics

OpenRA – which is open source reimplementation of Red Alert, Dune 2000 and C&C Tiberian Dawn - has solved this by offering to fetch the shareware game and extracting the assets from there. Fully libre distributable assets would offer even better experience, but this is one pretty neat way to handle the seamless start.

(It's not a reimplementation, it's a different thing that is asset-compatible with the originals. The gameplay, the balance, the engine are different, and the name of OpenRA is not fairly claimed IMO.)