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!