Minecraft - A new sandbox.

Here's what you get:

You never have to pay again!
You get all future versions of the game, including all expansions and addons.
You help fund the development of Minecraft
Custom skins in multiplayer
Once you've bought the game, it's yours. No DRM.
Survival mode!
Planned expansions:

Zombie Siege
Build your defenses during the day, fend off the incoming masses during night.
Capture the Flag
Defend your base and flag against the enemy team while trying to steal their flag.
Various custom level mods
Want to import a doom map? How about playing in a random dungeon?
And more!