I am mainly into Luanti [1], which actually is a game engine for Minecraft-like games but the community is slowly expanding so we are starting to see original games. Public servers run original games as well, which may or may not be technically open source.
In the same graphic style, there's Veloren [2] which is probably the most promising MMORPG-like FOSS game. This isn't Luanti, but a Rust project from scratch (not that I care about the implementation language, though, except when its poor performance becomes a limiting factor). Played some, but voxel graphics without a world fully editable by players makes less sense to me.
I used to play Cube2:Sauerbraten [3], which is an arena shooter. This is an old game, community is slowly shrinking. Was a good casual shooter when I played, but now probably the players that still play are probably accuracy monsters but I never was good getting old doesn't help. Offline vs bot is still fun, and there's a couple of campaigns in the box. Unvanquished [4] is an arena shooter as well with an original concept (played it a little a while ago, should check it out again).
Battle For Wesnoth [5] is top tier as far as turn-based strategy goes.
OOlite [6] is a recreation of the original Elite (Elite:Dangerous is the lastest Opus). Single player game, 600+ mods last time I checked. I was quite into it before moving to Luanti because my interests shifted. OOlite sort of runs at its own pace sometimes, that is the player doesn't have full control over the amount of "action" happening. It is also a difficult game, especially in the early game where it can be unforgiving if not unfair. IIRC, the game comes with a PDF file with some good piece of advice.
I sometimes play some SuperTuxKart [7] single player. Good for casual racing. There's also a football mode which is probably like RocketLeague, but that's not my thing. I used to play StuntRally [8] before the author rebased in on a more recent graphic engine after a long hiatus, which my rig cannot run for some reason. StuntRally has a lot of features - it had online mode before SuperTuxKart did.
0A.D. [9] is and AoE-like with top-tier graphics. Didn't play it a lot because I don't like much games when you mass-kill humans, especially women - call me old school.
In Zero-K [10] it's robots versus robots so it's fine by me. As an RTS, I find it in many ways superior to Starcraft except graphics, partly because it is tuned to handle a massive number of units (no population cap). It is based on the Spring Engine, which features pleasant unit positioning; recently I tried again Warzone2100, but the lack of this feature made it "unplayable" for me. Other games like Zero-K with better graphics are EvolutionRTS and Beyond All Repair, but I didn't play them because my rig can just barely handle ZK already.
I am mainly into Luanti [1], which actually is a game engine for Minecraft-like games but the community is slowly expanding so we are starting to see original games. Public servers run original games as well, which may or may not be technically open source.
In the same graphic style, there's Veloren [2] which is probably the most promising MMORPG-like FOSS game. This isn't Luanti, but a Rust project from scratch (not that I care about the implementation language, though, except when its poor performance becomes a limiting factor). Played some, but voxel graphics without a world fully editable by players makes less sense to me.
I used to play Cube2:Sauerbraten [3], which is an arena shooter. This is an old game, community is slowly shrinking. Was a good casual shooter when I played, but now probably the players that still play are probably accuracy monsters but I never was good getting old doesn't help. Offline vs bot is still fun, and there's a couple of campaigns in the box. Unvanquished [4] is an arena shooter as well with an original concept (played it a little a while ago, should check it out again).
Battle For Wesnoth [5] is top tier as far as turn-based strategy goes.
OOlite [6] is a recreation of the original Elite (Elite:Dangerous is the lastest Opus). Single player game, 600+ mods last time I checked. I was quite into it before moving to Luanti because my interests shifted. OOlite sort of runs at its own pace sometimes, that is the player doesn't have full control over the amount of "action" happening. It is also a difficult game, especially in the early game where it can be unforgiving if not unfair. IIRC, the game comes with a PDF file with some good piece of advice.
I sometimes play some SuperTuxKart [7] single player. Good for casual racing. There's also a football mode which is probably like RocketLeague, but that's not my thing. I used to play StuntRally [8] before the author rebased in on a more recent graphic engine after a long hiatus, which my rig cannot run for some reason. StuntRally has a lot of features - it had online mode before SuperTuxKart did.
0A.D. [9] is and AoE-like with top-tier graphics. Didn't play it a lot because I don't like much games when you mass-kill humans, especially women - call me old school.
In Zero-K [10] it's robots versus robots so it's fine by me. As an RTS, I find it in many ways superior to Starcraft except graphics, partly because it is tuned to handle a massive number of units (no population cap). It is based on the Spring Engine, which features pleasant unit positioning; recently I tried again Warzone2100, but the lack of this feature made it "unplayable" for me. Other games like Zero-K with better graphics are EvolutionRTS and Beyond All Repair, but I didn't play them because my rig can just barely handle ZK already.
[1] https://www.luanti.org/
[2] https://veloren.net/
[3] http://sauerbraten.org/
[4] https://unvanquished.net/category/news/
[5] https://wesnoth.itch.io/battle-for-wesnoth
[6] https://oolite.space/
[7] https://supertuxkart.net
[8] https://stuntrally.tuxfamily.org/
[9] https://play0ad.com/
[10] https://zero-k.info/