NBTExplorer is an open-source NBT editor for all common sources of NBT data. It's mainly intended for editing Minecraft game data. NBTExplorer supports reading and writing the following formats:

• Standard NBT files (e.g. level.dat)
• Schematic files
• Uncompressed NBT files (e.g. idcounts.dat)
• Minecraft region files (*.mcr)
• Minecraft anvil files (*.mca)
• Cubic Chunks region files (r2*.mcr, r2*.mca)

More details can be found here