The map editor is a Windows program that edits the gamesave of the Nintendo DS game "Animal Crossing:Wild World" (or the Japanese version "Oideyo Dobutsu no Mori").
NOTE: You must buy extra hardware so you can backup your game save from the NDS to your PC. If you do not already have a Homebrew capable NDS, please wait.
Action Replay (for GBA+DS, $35) is a cheaper alternative. Unfortunately it does not currently support the AC:WW game (firmware 1.06). If/When they add support, you can use it with the PC Map editor (you don't need the NDS trainer app).
With the map editor, you can customize your inventory, or add additional money ('Bells'). You can customize the world map, placing rocks or trees wherever you want. Also you can leave items laying around to pickup later. You can customize the rooms of your house in traditional and not-so-traditional ways.