Minecraft, the most famous game.

Honestly, i'm not surprised by the fact that Minecraft itself was able to grow with an enormous of playerbase. It all started off with a game that's not very well known and was created by a extremely smart guy. Notch was able to gather tons of playerbase without doing a lot of advertising.

The game itself is a 3D sandbox game, allowing players to build their own imagination into a game. With plain building blocks, mobs, pvp, etc. I'm just surprised how it can be so successful!