Kyle sent out this amazing tool for testing and debugging at the HTTP level. This would've been a lifesaver a few weeks ago when I was working on RESTful API stuff for work, but better late than never.
The idea of the app is amazingly simple. It lets you specify the type of http request you want to make, the header options, and the body. The beauty of doing it in a GUI instead of from the command line with curl is that there are dropdowns for the hard to remember header options, and that you can paste fat hunks of XML into the body field. It's also awesome because you can open multiple windows and have each of those be a re-runnable http client.
HTTP Client is 99% perfect app for my uses. The single feature that would push it to being a perfect app would be a cookie-jar so that sessions can be saved.