Cost money:
Minecraft - 'nuff said. I'll assume you know about this one. 😛
Don't Starve - Awesome survival/crafting game with a neat art style
Dungeons of Dredmor - Fun roguelike with a cute sense of humor and an approachable interface!
Rogue Legacy - Roguelike metroidvania in which instead of collecting loot along the way and dying horribly and having to completely start over, you collect money and unlocks, and your character's descendants get to use them!
Bastion - Really neat isometric ARPG with great atmosphere.
Awesomenauts - Approachable 2D platforming action-packed MOBA (DOTA-like) game with a community that won't generally treat you like garbage if you're not super-pro at it.
Space Pirates and Zombies - 2D space exploration/combat/etc. game in which you go around blowing stuff up and building up your fleet, lots of fun, procedural generation, and tons of replayability.
FTL: Faster than Light - Starship roguelike! You control a ship with a crew, systems, etc. with everything cool you'd hope for from such a game, like being able to re-route power on the fly, send out boarding parties, use unique and fun weapons, and most importantly, die frequently in often hilarious ways!
Lots of games by Valve! Portal, the entire (main) Half-Life series, Left 4 Dead 2, DOTA 2, Natural Selection 2, and more on the way!