Not many games will allow you to build a flying Steampunk fortress, while you brace for impact as a massive flying Kraken smashes it to pieces - Airships: Conquer the Skies let's you do that.
The Communist Dogifesto has quite an amusing setting, with Russian space dogs aboard a space station becoming really rather intelligent and taking it over. It's open source and it just had a rather big update.
If you fancy learning a new language, Lingotopia might make it less of a chore as this adventure game has you play as a little girl shipwrecked on the shores of a strange island.
Dicey Dungeons is a new roguelike from Terry Cavanagh, Marlowe Dobbe, and Chipzel that seems quite simple, until you start getting further in and it becomes quite addictive.
Sadly we missed the actual release for the really sweet couch co-op adventure 39 Days to Mars, but we're making up for it by letting you know it has some additional free content!
For those of you who love your idle/clicker games, you might want to take a look at Chicken Assassin: Reloaded as it now has official Linux support, it seems quite popular too.