Why it matters: Gamers might be surprised to learn that there is a thriving open-source game community. Not only are these games completely free, but they can run on a potato and are compatible with multiple platforms, including Linux.
While many of today's AAA titles are free-to-play, they come with insidious pay-to-win models where gamers are lured in by microtransactions and loot boxes. Indie titles offer a minor reprieve from these greedy cash-grabs, but free-to-play, in the strictest sense of the term, only exists in open-source games.
You'll find a wealth of resources online, and a good place to start would be the GitHub Stars Top 50. As the name implies, this is a list of the highest rated open-source games on GitHub. The full list includes over 1,300 games.
Most of these games are small, stripped-down clones or remakes of old classics, and are sure to offer a pleasant nostalgic trip for gamers. OpenRA attempts to recapture the Command & Conquer games from the 90s, and with OpenRCT2 you can travel back to the glory days of RollerCoaster Tycoon 2. But there are also more modern titles such as Craft (Minecraft) and Unciv (Civilization V).
OpenRCT2, inspired by RollerCoaster Tycoon 2
Developers and modders will find these projects equally appealing. Open-source means the source code is freely available on GitHub, so developers can use it as a springboard for their own game. With most of these projects running under the GPL or MIT licence, the code can be modified and redistributed. Some projects even encourage collaboration, where developers are free to submit a pull-request which may be included in the next version of the game.
For another comprehensive list of clones and remakes, head on over to Open Source Game Clones. By clicking on the "repo" link, you will be taken to the respective game's GitHub page, where it can be downloaded. Dos Game Archive has some gems from the 80s and 90s for those chasing that nostalgic feeling only a retro classic can provide.
I think most gamers have a few precious old school titles that they revisit from time to time. As for me, time to fire up some Freedoom.