Great suggestions in this discussion! Rather than adding my favourites, I will add some resources that list more games.

• Libregamewiki: it is really comprehensive (sometimes too much, including even not-so-good-games). They care about licencing and is is very easy to browse, top-notch for me.
• Open source games: a more relaxed repository, with lots of material.
• bobeff open source list: this is curated, which means that there are not so many games but each and every one is stable, good, maintained.
• Arcane Cache: a fantastic blog with reviews of libre games — or more precisely, underground games, there is a lot of discussion on how gamedevving philosophy too. The reviews are always in-depth and allow you to experience the games on another level, and each game is a small jewel in its category. Strongly recommended!

A lot of good tips here, but you should have a look at Endless Sky! Really cool space game
https://github.com/endless-sky/endless-sky

Havent seen endless sky mentioned here, it is an old school space trader sim (think scape velocity or a 2d freelancer)

Shattered Pixel Dungeon Roguelike, very addctive :)

• Xonotic is a great old school, Quake style FPS with an active community
• 0 A.D. is a pretty polished historical RTS
• The Battle for Wesnoth is a classic turn based strategy
• OpenRA is an open implementation of the Red Alert engine that's got an active community
• Openhv is an OpenRA based sci-fi RTS

NetHack - Roguelike

Space Cadet Pinball - Decompilation of 3D Pinball for Windows – Space Cadet

The Battle for Wesnoth - Turn-based strategy game with a high fantasy theme

Mindustry - Sandbox tower-defense game

Minetest - Voxel game engine