Minecraft Building Tools and Resources
From Aporkalypse Wiki
If you want to build complicated things, you can construct them in Google Sketchup before building in game. More info at the Minecraft forum thread
If you want to build a giant sphere, rov4416444 made a python script that generates a hollow shell sphere. Requires Python.
If you're the planning sort, here are some resources to help plan your structures before you build them.
