Tetris I suppose, but that kind of is the low-hanging fruit. Especially since it was basically made by one person who has gone on to be...pretty capitalist.
Disco Elysium is probably the best example, but the developers were basically ousted and the license was usurped. I would suggest finding alternative ways to get the game that won't profit the usurpers.
Dead Cells used to be developed by a workers collective (Motion Twin) though the team that took over (Evil Empire) is not a coop. Motion Twin was still involved in the Evil Empire projects as far as I am aware.
Stray Bombay (The Anacrusis) is organised as a co-op, I think.
