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.
- NEINedit: NBT editor for Mac by G-Man.
- INVedit: Inventory editor by me.
- Inside Job: Inventory editor for Mac by preble.
- Too Many Items: Inventory editor mod by Marglyph.
- Not Enough Items: Inventory editor mod and more by ChickenBones.
- MCEdit: World editor (originally by codewarrior, now open source and maintained by TkTech).
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