I just wrote a bit of code using Groovy HTTPBuilder to fetch and parse some JSON content to help answer someone's question on StackOverflow. When trying to pull some interesting data out of the JSON to display, I realized that it would be much easier for me to understand the structure of the JSON if it were pretty-printed (or pretty-formatted). When I googled for "JSON pretty format", I found a helpful blog entry pointing me to Curious Concept's JSON Formatter (& Validator). It's a simple online tool that allows you to paste in either a URL pointing to JSON content or the content itself and display it in either compressed or readable form. It might not have been very hard to code, but it's extremely useful. Thanks Curious Concept!
Update: I'm trying out the JSONView Firefox plug-in. So far it seems to work pretty well. Even so, I expect I'll continue to use Curious Concept's tool for a while, since I spend a fair amount of time in Chrome.