It might surprise you to learn that there are very many free games that don't fit the modern free-to-play model. I say that because every time somebody posts a link to some impressive, public, open-source game project that minutely recreates some abandoned classic people seem shocked.
So here's some of the best, the big fat lists of open source games and open source game clones. What do I mean when I say that?
I mean these are free public projects that produce games inspired by and using the mechanics of old classics, games like Rollercoaster or Transport Tycoon, Doom, XCOM, Red Alert, Civilization, and Sim City—not to mention the classic roguelikes like NetHack and Dungeon Crawl Stone Soup.
First off I'll point you at the Open Source Game List, or OSGL, a lovely list of these games. Specifically their Top 50 Open Source Games is an invaluable resource for people getting started on the cool old stuff. It'll point you at classics like OpenRCT2 and GZDoom, as well as impressively unique masterworks like Mindustry and Citybound.
My other go-to is osgameclones, which is a frankly overwhelming and vastly tagged list of games. It's a bit tricky to get the hang of, but has tags you can click on to understand what's going on: Tags like Clone, Official, and Remake help differentiate what's going on. For those who want to really wade in, I suggest the complete and playable tags as strong starting points.
Finally, you might enjoy libregamewiki, which is exactly what it sounds like: A big ol wiki of open source games.
These resources are not only cool adventures in programming, they're also lovely work that helps people gaming on low-end and low-spec systems. Lots of them are also workable on or have a Linux distribution, for all you newly-minted Linux users who've come to the OS via Steam Deck. I've got a real hankering to grab Freeciv and OpenXCOM now, frankly.
I'll see you later. Probably.