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!
Colobot was made open source a while ago and is still great! The game assets (and original binaries) can also be downloaded for free from the dev's site.
Cataclysm: Dark Days Ahead, or if you like something a little less complex, Cataclysm: Bright Nights, are fantastic roguelike games. Proper traditional, turn based, tileset ones.
Imagine a mix of Fallout and The Walking Dead, that's what it's like. It's superb. The possibilities are endless
Forge, a Magic: The Gathering rules engine available both for Windows and Android. Various formats, good AI, offline and online play, and now even has a Shandalar-like mode. #MTG#MagicTheGathering#Forge#Shandalar
Add comment