Here's my list of current free open source games that can be played on Windows/Android.
Any Steam links are to the paid versions, but free local standalone versions are available for all of these games.
Most are listed in the Google Play Store, but a few APKs are buried in their respective websites/wikis.
Flare is an open source, 2D action RPG licensed under the GPL3 license. Its game play can be likened to the games in the Diablo series.
Freeciv is a Free and Open Source empire-building strategy game inspired by the history of human civilization.
Mindustry: A sandbox tower-defense game.
Minetest is a free software game engine to create various games based on voxel gameplay, inspired by InfiniMiner, Minecraft, and the like.
Pixel Dungeon is a traditional roguelike game with pixel-art graphics and simple interface for Android, iOS, Windows, Mac and Linux.
Shattered Pixel Dungeon is a traditional roguelike dungeon crawler that’s simple to start but hard to master! Every game is a unique challenge, with four different heroes, randomized levels and enemies, and hundreds of items to collect and use.
The Battle for Wesnoth is an open source, turn-based strategy game with a high fantasy theme. It features both singleplayer and online/hotseat multiplayer combat.
Unciv: An open-source reimplementation of the most famous civilization-building game ever - fast, small, no ads, free forever!