Minecraft modder SethBling has built an interpreter in the BASIC programming language, which essentially lets gamers compile and run scripts while in-game. So you're using Minecraft's voxel world to make and run small, simple little programs. Users can do a variety of nifty things with SethBling's BASIC interpreter, including solving everyday calculations or counting prime numbers to infinity.
For a more practical solution, users can write simple scripts to programming automated mining turtles--basically little bots--to do various tasks. These mining turtles can dig holes and collect materials all day while you're AFK, making for a nifty little addition that can save you quite a bit of time.
Of course, the BASIC commands are a bit tricky and require some know-how to use. The game's low-res voxel-based world also resists this kind of computational input: thanks to Minecraft's 20hz clock, you can never perform more than 20 commands a second, making some scripts rather slow.
Even still, it's just amazing to see someone write and execute programs within a game like Minecraft, and we can only guess what these talented coders will turn up next.