Making huge, complex builds has been a staple of Minecraft forever. I don't know about you, but for me, one of the hardest parts of a large build has always been planning - knowing exactly where you're going to build, how large it will be, and where everything will go. For people like us, there is now a really sweet mod, called "Schematica", created by Lunatrius!
With this mod, you are able to display a ghost image of a schematic file inside Minecraft to rebuild a structure, or export any part of your world (or a server's world) into a schematic file. You're able to export parts of your world, too! Gone are the days of random builds, using MS Paint to plan, or spending hours on a build, only to discover you were ONE BLOCK OFF.
WHERE TO GET THIS MOD:
Take note: this mod requires Forge, as well as another mod made by Lunatrius, which serves as a hub for all of his mods.