My Twitter OAuth ebook closes with the source code for an API Console application. This app got such a favorable response that I decided to enhance it and put it out as a free tool. I have found this to be an invaluable debugging aid when testing an API request. It lets you enter just the description of the request and quickly see the complete response without having to write any test code.
The tool is pretty obvious to use, but I also added a users guide to cover some of the features you might not notice, such as the ability to share the URL for an API request. This lets you easily demonstrate a bug in the API to Twitter HQ, or show a fellow developer how to do a specific API task.