Minecraft Forge - Minecraft Forge is a free, open-source modding API and loader designed to simplify compatibility between community-created game mods in Minecraft: Java Edition.
WorldEdit - Minecraft MOD - WorldEdit is an easy-to-use in-game Minecraft map editor. Through a combination of commands and "brushes," you can sculpt your world or simply perform numerous terraforming tasks.
Wurst - Minecraft open source hack
MCEdit - MCEdit is an open source world editor for the popular game Minecraft.
Schematica - Minecraft MOD - With this mod you're 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!
WorldGuard - Minecraft Plugin - WorldGuard is a powerful plugin with a large bag of tricks for server owners, server map makers, regular survival servers, and everyone in between!