Kerbal Space Program - like space and building things? Buy it. Have over 160 hours into it so far, and have only been to 4 planets.
Prison Architect - Don't let the price tag put you off, it's worth every penny.
Binding of Isaac - It's the original Zelda, but in a basement dungeon full of the strangest things you'll ever see, with 100x the gear.
Space Pirates and Zombies - Awesome spaceship fighter; like Star Control but more action focused. Highly recommended.
Terraria - Minecraft in 2D graphics, with an actual game behind it. Tons of craftables, huge boss fights, and one of the best co-op indie games.
Defender's Quest - RPG/Tower defense that is quite fun, but does entail some serious grinding late game. Still, for $2.50 (sale), great game.
FTL - Great space ship strategy fighter. I'm convinced any Star Wars/Trek game should ripoff the large ship combat from this.
Legend of Grimrock - Old school RPG dungeon delving with modern twists.
Spectromancer - If you like card type games like Magic, then you might like this.