I love HTTP. Here are some of my favorite tools for working with it.
Hurl.it makes HTTP requests from the browser. Inspired by curl and originally built by Leah Culver (Convore, Grove.io) and Chris Wanstranth (GitHub) but now maintained by Twilio. Hurl.it is awesome for testing out APIs, checking to see if an issue is just on your system or anything else that requires an HTTP request. The code is on GitHub and you can follow on Twitter for updates.
Jeff Lindsay aka progrium recently released a beta of RequestBin, his revamp of Postbin.org. RequestBin is the opposite of hurl.it. It gives you a URL and shows you a dump of all the requests sent to those URLs. The latest version has an API and private bins.
Kenneth Reitz of Python requests fame (the best Python HTTP library by far) has put together a utility for testing HTTP clients. You can easily generate responses that return specific status codes, echo back request data, require authorization, etc. A similar service from Aaron Powell and Tatham Oddie is httpstat.us which returns different status codes based on the URL requested.