There is a Wikipedia article about this: https://en.wikipedia.org/wiki/List_of_open-source_video_games
It is, however, vastly incomplete, as entries without “reliable sources” get deleted. Mind that linking the source code repository, the steam page, the license file and news about a game going open source are not enough to count as “reliable source”.
Technically I’m still playing “Vagrus - The Riven Realms”, but I didn’t play much lately, since I rediscovered my love for the Lean4 programming language and am now playing around with a formally validated heap again.