This tool is very old, and I don't even have the source code of my old NBT library which I used in this project. I don't want to, and also can't update NBTedit to add support for the new int[] tag which was added to Minecraft a while ago. There's a few other tools linked at the bottom of this post which might be helpful (I suggest NBTExplorer), and if you find / made any I'd be glad to link them.
Happy minecrafting.
I made this some time ago, but never actually released it.
Anyway, with this beauty you could edit every file in the NBT format (see specs here). But afaik there are no other projects which are using this, yet. Most of you will probably use NBTedit to cheat items in Survival.
Here's some screeny:

You can simply edit bytes, shorts, ints, longs, floats, doubles and strings. Byte arrays must be exported and imported to edit them (I couldn't think of an easy solution). You can add, rename and delete Tags as well as search for a specific name, value and TagType.
Made with SharpDevelop (C#) and compiled for the .NET Framework 2.0.
.NET 2.0 or Mono is needed to run it.
Download: Click me! (Mirror: Mediafire)
The current version is 0.9.8.

Check out the wiki article on Data values and the level.dat Format!
Other helpful tools you may want to check out:
- NBTExplorer: A better NBT editor by jaquadro.
- In-Game NBTEdit: Forge compatible mod for editing (tile) entities by Davidee.
- INVedit: Inventory editor by me.
- Not Enough Items: Inventory editor mod and more by ChickenBones.
- Too Many Items: Inventory editor mod by Marglyph.
- MCEdit: World editor by codewarrior.













