I did some looking around for something to parse and view JSON for me. I found JSON Viewer from CodePlex. It is really nice. However, it is Windows only. I do some development on Windows, but a lot on my MacBook as well. I wanted something that would work on either.
Instead of spending more time looking, I decided to code. A year ago I might have written something in Java, but I decided this was a good chance to write something in ActionScript and leveraging Adobe AIR to create a desktop application. The result is an AIR-based JSON Viewer. It looks really ugly, but I'm no designer. It uses Adobe's corelib to parse JSON and then just massages the resulting object into a structure that can be read by a Flex Tree control.
One fun use of it is to go to some of your favorite sites. Open up Firebug to watch traffic. Look for some JSON, plop it into the app and take a nice look at the data being sent over the wire to your browser (or from your browser in some cases.) Of course some foolish sites still use XML over the wire, but most of the bigger and/or newer ones use JSON.