Tool to Unminify / Decompress JavaScript

You can use this : http://jsbeautifier.org/ But it depends on the minify method you are using, this one only formats the code, it doesn't change variable names, nor uncompress base62 encoding.

edit: in fact it can unpack "packed" scripts (packed with Dean Edward's packer : http://dean.edwards.name/packer/)

Chrome developer tools has this feature built-in. Bring up the developer tools (pressing F12 is one way), in the Sources tab, the bottom left bar has a set of icons. The "{}" icon is "Pretty print" and does this conversion on demand.

UPDATE: IE9 "F12 developer tools" also has a "Format JavaScript" feature in the Script tab under the Tools icon there. (see Tip #4 in F12 The best kept web debugging secret)