HttpMaster - HttpMaster is a professional software tool for testing and debugging HTTP applications, primarily aimed at REST API applications and web services.
openDesktop.org - The website openDesktop.
MockServer - Easy mocking of any system you integrate with via HTTP or HTTPS.
SourceForge - The Complete Open-Source and Business Software Platform.
Request inspector - Debug web hooks, http clients
Eclipse - Eclipse is an open source community, whose projects are focused on building an open development platform comprised of extensible frameworks, tools and runtimes for building, deploying and managing software across the lifecycle.