There are a lot of games that can no longer be played because the systems to run the games are no longer in production and it is illegal to modify their code to work on the new systems or operating systems or emulators. That is where open source comes into play, developers have dedicated a lot of time creating open source clones of their favorite games.
You can access the list and instructions on how to install/play them at: https://osgameclones.com/, which gathers open-source or source-available remakes of great old games in one place.
A Remake is a game where the executable and sometimes the assets as well are remade open source. Some of these games aren’t exact remakes but evolution of original ones, which were eventually open sourced.
A Clone is a game which is very similar to or heavily inspired by a game or series.
An Official project is the official source code release for a game that was formerly closed-source, maintained by the original creators and has minimal changes.
A Similar game is one which has similar gameplay but is not a clone.
A Tool is not a game, but something that assists in playing or modding the game, such as a high resolution patch, or resource extractor.
I see Open source versions of Classics like Decent II, Doom II/III and many more on the site. Check it out if you have some free time.
Source: Boingboing.net: Open source clones of classic video games