Recommended, but not here yet

These need their own game files, a server, or I haven't finished checking them. Every link goes to the project's own page. I don't copy them.

Lichess: Free online chess against real people. It has no ads. Players: 2+ online. Licence: AGPL-3.0
Doom (shareware episode): The first Doom episode through the Chocolate Doom engine. Players: 1. Licence: Engine is GPL. Only episode 1 data may be shipped (id shareware terms).
Anarch: A tiny Doom-style shooter released into the public domain. Players: 1. Licence: CC0 (check repo)
Blobby Volley 2: Volleyball with blobs. Players: 2 on one keyboard. Licence: GPL (check repo)
Bombermaaan: A Bomberman clone for a crowd. Players: up to 6 on one keyboard. Licence: GPL (check repo)
Jump 'n Bump: Bunnies jump on each other's heads. Players: up to 4 on one keyboard. Licence: GPL (check repo)
C-Dogs SDL: Top-down shooter with co-op. Players: 1 to 4. Licence: GPL (check repo)
Chromium B.S.U.: A vertical space shooter. Players: 1. Licence: Artistic licence (check repo)
AstroMenace: A 3D space shooter. Players: 1. Licence: GPL (check repo)
Brogue CE: Roguelike dungeon crawler. This is the community edition. Players: 1. Licence: AGPL (check repo)
Angband: A long-running roguelike set in a Tolkien-style world. Players: 1. Licence: GPL (check repo)
Mindustry: Factory building and tower defence. Online co-op needs a server. Players: 1 or online co-op. Licence: GPL-3.0 (check repo)