MCEdit is a Minecraft saved game editor, which can also edit Minecraft worlds.
In Minecraft, the player takes on the role of a world explorer, miner, and monster slayer as days and nights pass, countless things happen, and the vast expanses of the randomly-generated world around them continue to beckon them to new and brighter pastors... or, the player might simply decide to build a mansion, cobble together everything good in the region, and hunker down, living a relatively idyllic life. Minecraft is a "sandbox game", where your goals are the ones you create, and there's tons of things to do in it.
Naturally, then, there's a lot to do with a Minecraft saved game. A player's saved game also includes the world they live in. Minecraft worlds have mountains, beaches, and plains; they have places high in the clouds or deep beneath the earth's crust. Fabulous items can be found in unlikely places, or created from the minerals a player might find during the underground treks.
What, then, if a player wanted to find minerals in their above-ground treks, or simply have them? What if a player would much rather have a Diamond Sword than diamonds? A saved game editor like MCEdit is useful for this purpose. It's useful, too, if you'd like to build a house out of rare diamonds, or simply to make a change to an existing house that would be difficult or impossible while inside the game.
MCEdit's functionality is almost perfectly designed. From a top down view of the world, a player wanting to edit it can take almost any point of view they need while editing it. While "zoomed in" to the world, editing it from a first person perspective, the interface is a lot like the Minecraft game itself, just with administrative tools instead of the kind you'd use to play. Blocks can be placed or removed, foes created or destroyed, and screenshots taken of beautiful topographical views of places in your world.
MCEdit is missing very few functions, and can be recommended to any Minecraft player who wants to tweak or edit their saved game. The only disadvantages are that it is currently undergoing a rewrite, so later versions may change significantly after you've learned the program, and that it lacks full compatibility with Minecraft Pocket Edition.
Even then, though, there is enough there to edit your Pocket Edition saved games in MCEdit – and if you have a Minecraft "classic edition" saved game, you can export it to other Minecraft gameplay modes with MCEdit as well. MCEdit comes highly recommended to any Minecraft player looking to do heavy editing, of character or world!