Another great way is via the json ruby gem, which installs a prettifier which can be used from the command line, like this:

    curl 'http://api.twitter.com/1/statuses/user_timeline.json?screen_name=twitter&include_rts=1 | prettify_json.rb| less

Another way to do it without having to install anything extra:

curl "http://api.twitter.com/1/statuses/user_timeline.json?screen_... | python -m json.tool | pbcopy

I use the Chrome extension JsonViewer. It automatically formats the json. Great for api inspection.