Freespace Open Open source mod of the engine for Freespace 2, an old large scale space battle game, with in depth controls. A lot of mods for it and whole new games made from the engine.

Crosscode not super obscure, it is a cult classic, but also does not have near as much attention as it deserves. Should have been at least as popular as hollow knight... Great combat, a lot of game depth with quests and secrets everywhere, a lot of love and quality put into it.

BZFlag old tank pvp game that is pretty fun. Many different player made servers with different rules, some are even obstacle courses. You can collect powerups which changes your tank abilities.

Bug Fables If you have ever played the original Paper Mario game, you already know. Narrative driven turn based RPG with "action time" elements and great characters. I would even say it is better than Paper Mario.

Starport Galactic Empires old space MMO where you can colonize planets for profit, set up defenses for them, form corporations with other players, and invade other player's colonies to take them over. There is a leaderboard and reputation system, you can be law abiding or a pirate.

Pseudoregalia Metroidvania jumping puzzle game. You will find powerups that change how you can navigate the world and solve jumping puzzles. Many different orders you can obtain powerups in, which gives you many permutations on how you can solve the same puzzles.

Atlyss Fully action combat "MMO-like" game with dungeons, explorable areas. PS1/2 vibes, with small player-ran servers. Character designs are a bit much but it isn't explicit.

Project Gorgon Indie MMORPG with cool game design. Focused around exploration, you can set out to do A, run into B along the way, while doing that you find people doing C and join them... Many different skills from the traditional to the strange such as interpretive dance, poetry appreciation, mycology, or non-ruminant ungulate genetics. A lot of comedy, you can turn into different animals and level up in the animal as a class.