I've given some thought to making a better world editor that would interface with Minecraft through command blocks or via the Glowstone MC server. There are a few existing world editor applications but they are quite sluggish (last time I checked). If you want to build small things with cubes then you should check out the latest voxel editors.
I checked some voxel editors sometime ago(unfortunately not remembering their names right now), they seemed geared towards modeling small things, not huge monuments, vistas, mountains or expansive landscapes like people seems to do in minecraft.
