Start from Doom. Well, not quite the original release, but rather Doom Legacy. From this base, create a 3D Sonic platformer styled after the Genesis era games. That’s Sonic Robo Blast 2 (because yes, it’s a sequel from a previous fangame named Sonic Robo Blast from all the way back in 1997).

We’re not done yet. Mod it further… into an online kart game. That inherits the same modding abilities Doom Legacy has. You’ve got Sonic Robo Blast 2 Kart, sometimes nicknamed MUGEN Kart to the dismay of its developers due to the sheer amount of addons for increasingly weird characters its forum hosts. If you’re interested, said forum, hosting both the addons for SRB2, SRB2 Kart and the SRB2 Kart releases themselves can be found here (for some reason the main website is down as I’m writing this. Oops.)

For SRB2 Kart in particular, the thread to get your hands on it is here and you can find its repository here.

The game is wacky, very fun, runs on a stale potato and it’s easy to set up a server to subject an unsuspecting community to your specific choice of insane mods among the hundreds if not thousands available.

Shattered Pixel Dungeon the most active fork of pixel dungeon, a rogue like dungeon crawler:

Pros: Simply fun

Polished AF

Has a lot of well balanced items, enemies and unique classes/sub-classes (giving it infinite replayability) actively updated

Also has a lot of great and unique forks (commonly called “Mods” by the community) such as Rat King Adventure

Has good support for both PC and phones (I also think it works well with controllers)

Cons:

You will die

You will die a lot (don’t worry, it’s worth the pain)

The game that I probably go back to the most frequently is Cataclysm: Dark Days Ahead. While it looks like any other roguelike on one level, it has a bunch of different interacting systems, story elements scattered around (if you only find one cataclysm, then you’re not looking hard enough), and has a debugging system that (if you’re not interested in the action-oriented aspects) can be used to cheat and turn the game into exploring the randomly generated world.

My personal favourite is Dungeon Crawl Stone Soup, although it’s quite tricky to get into and still brutally difficult, even if you are in it.

It’s similar in concept to Shattered Pixel Dungeon, which others have mentioned here. Maybe try that one first. 🙃

Shattered Pixel Dungeon is currently the one that has me gripped most.

Apart from that it’s probably those derived from id engines. Last one I played was Jedi Outcast.

It’s OpenTTD, hands down. I think source ports and game reimplementations are where open source shines its best.

Here’s my favourites:

Blob wars is a 2d platform/shooter that is great fun (great sound effects and hilariously excessive gore).

Warzone 2100 is an RTS with a “neoclassic” appeal (something that would have felt like “the future of RTS” back when you played Command&Conquer).

Battle of Wesnoth is a hex-grid strategy games with loads of mods and campaigns.

looks like nobody mentioned Warzone2100 yet. SciFi rts. I don’t know how many hours I lost to OpenTTD. Bitburner was FOSS too, I guess. I might check my library later to extend the list…

I love Mindustry

I’ve played a decent bit of 0ad over the years, I’m a big fan of Thrive, and NZ:P is a pretty neat open-source PSP clone of CoD’s Nazi Zombies.

Battle for Wesnoth is really good.

I like simutrans a lot (maybe the most elaborated TTD-like transport game), and I am trying to learn freeciv, but it is a bit “technical”. I also like minetest, I have constructed a lot of big houses, the only FOSS game where you can cook a lot of recipes.

Simon Tatham’s Puzzles, particularly Net on a very large board with wrapping.

I also like good ol’ Stepmania (or the more recent ITGmania fork), but I’m pretty bad at it.

OpenTTD
The commercial original was already a timeless classic and the open source remake is just that but more of it.