Yesterday I talked about JSON, and the inspect() method I use. While that example actually did some nice indenting, sometimes I have a mass of untabbed JSON I'd like to visually inspect. The best tool I've found for that is http://json.parser.online.fr/ . Slightly confusingly it shows both the "String parse" and the "JS eval" versions of the string, but usually it doesn't matter which one you look at. The way it builds a treeview of the code is really nice.
Before finding that page I used to just Google for "json pretty print" which would lead me to Cerny.js' entry as well as curiousconcept's. Both are ok in a pinch, I think the former sometimes got confused by special characters in the content, even if they were properly escaped.