Minecraft is a game about placing blocks to build anything you can imagine. At night monsters come out, make sure to build a shelter before that happens. It also has music byC418! So far 8,453,864 people have registered and 2,324,712 people bought the game.
The Game is currently 25% off if you are interested in buying it.
The private server community is just as large though. (This allows you to play online after downloading the game for free off the internet)
There are also many add ons to this game such as COD Nazi Zombies, and the original game of Portal.