Postman is a great tool to play with APIs such as Twilio's and even to test your own. It comes in the form of a Chrome application, and lets you build, create, and save HTTP requests, keeping track of what you sent and the response you got back.

To get started, you'll need to make sure that you have Google Chrome installed (http://chrome.google.com), and then you can download Postman from Chrome's Web Store at https://chrome.google.com/webstore/detail/postman-rest-client-packa/fhbjgbiflinjbdggehcddcbncdddomop?hl=en.

Once installed, you'll be able to use Postman from Chrome's Apps screen. It'll also be installed as a standalone application that will be available from your Applications folder and Spotlight on Mac OS X, or from the Start menu on Windows.