MCEdit is an open source world editor for the popular game Minecraft. MCEdit was first created to allow players to preserve anything built with several old versions of Minecraft and take them forward into newer versions of the game. It also aims to be forward-compatible with future (or even modified) versions of Minecraft. It has since been improved with brush tools for laying down blocks in different shapes, integration with the Minecraft Server to generate terrain using Minecraft’s own seed algorithms, support for multiplayer worlds, and editors for certain blocks including chests and mob spawners.

Features:

Navigate the world using familiar WASD controls and mouse aiming.
Show the locations of dropped items, game entities including monsters and villagers, hidden ores, plus blocks with TileEntities and other Minecraft internals.
Load saved games from current Minecraft versions, plus the older Classic and Indev versions.
Limited support for Minecraft Pocket Edition saved games
Export blocks and game entities as a .schematic file to be imported by MCEdit or compatible programs.
Automatically fixes wool colors when importing and exporting between Minecraft PC, Classic, and Pocket editions!
Double-click on a chest to change its contents, or double-click on a mob spawner to choose what spawns.
Brush tool “paints” blocks in a round, diamond, or square shape with configurable size. Different brush modes will add new blocks, replace blocks, alter the top layer of the terrain or wear it down to smooth out rough objects.
“Paste” brush allows you to import an object with a single click.
“Flood Fill” brush will change all blocks that form a connected mass.
Clone tool makes a copy of blocks and entities, optionally making multiple clones in a line or scaling the copy up or down.
Clone and Import tools can rotate an object around any axis, or flip it horizontally or vertically.