NBTExplorer is an open-source NBT editor for all common sources of NBT data. It’s mainly intended for editing Minecraft game data.
You should be familiar with this tool if you handle Minecraft maps. Here you can open the level.dat file and change some parameters. For example change the game mode, difficulty, or activate map structures (useful for the nether and the end, but before doing this, use worldborder to pregenerate the overworld. More infos in the next blogpost)