toxy is a fully programmatic and hackable HTTP proxy to simulate server failure scenarios and unexpected network conditions, built for node.js/io.js.

Cliquet is a toolkit to ease the implementation of HTTP microservices, such as data-driven REST APIs.

Apache CouchDB™ is a database
that uses JSON for documents,
JavaScript for MapReduce indexes,
and regular HTTP for its API

RequestBin gives you a URL that will collect requests made to it and let you inspect them in a human-friendly way.
Use RequestBin to see what your HTTP client is sending or to inspect and debug webhook requests.

Record your test suite's HTTP interactions and replay them during future test runs for fast, deterministic, accurate tests.

Hurl.it makes HTTP requests.

Choose the request method, customize headers and POST parameters, add basic authorization or OAuth credentials, and even follow redirects. Then view the nicely formatted request and response.

It's the perfect tool for testing APIs. Just enter a URL and click send.

Testing an HTTP Library can become difficult sometimes. PostBin.org is fantastic for testing POST requests, but not much else. This exists to cover all kinds of HTTP scenarios. Additional endpoints are being considered (e.g. /deflate). All endpoint responses are JSON-encoded.

Configurable HTTP/REST Client Library for Python

An article on the best practices to apply in order to minimize HTTP round-trip times

mitmproxy is your swiss-army knife for debugging, testing, privacy measurements, and penetration testing. It can be used to intercept, inspect, modify and replay web traffic such as HTTP/1, HTTP/2, WebSockets, or any other SSL/TLS-protected protocols. You can prettify and decode a variety of message types ranging from HTML to Protobuf, intercept specific messages on-the-fly, modify them before they reach their destination, and replay them to a client or server later on.

A free web debugging tool

An HTTP Viewer and HTTP Sniffer for IE and Firefox