Single Player Commands is a very popular mod. It allows you to access a ton of commands to work on single players including World Edit. Since the 1.3 update merged multiplayer and singleplayer together, it is really hard for simo_415, the developer, to update this mod. He is slowly recoding this mod and hope that it would work in lan and eventually multiplayer.
How to download and install Single Player Commands for Minecraft 1.3.1
- Download single player commands
- Drag files into minecraft.jar except WorldEdit.jar
- Drag WorldEdit.jar to bin folder
- Delete META-inf