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- Beacon Pines -- a charming mystery story with anthropomorphic animals. Has an interesting take on the visual novel formula by having you unlock new dialogue choices as you progress through the story; that way, you naturally explore different paths the story might take. Night in the Woods and (possibly) OneShot fans might like this one.
- Oolite -- a solid FOSS remake of 1984's Elite. Has a bunch of mods for it; some expand the gameplay quite substantially.
- Orbiter Space Flight Simulator -- imagine a Microsoft Flight Simulator game, but you're going to space instead. Or Kerbal Space Program, but without the rocket building mechanic. That being said, KSP fans (and fans of space in general) should enjoy it.
- Transcendence -- Star Control II meets Rogue. A cult classic in the space sim genre that's been in development since 1995. Space dogfighting, trading, mining, smuggling etc, but also traditional roguelike stuff like unlabeled barrels and containers (= undiscovered potions) and permadeath (optional). Highly moddable, uses XML as the modding language. Has a free version (see link) and a Steam release, which includes the paid expansions.