soapUI - SoapUI Pro is one of the most prominent API testing platforms around, allowing developers to quickly prototype the functions of their apps and get them to market with little hassle.
Charles Proxy - HTTP proxy / HTTP monitor / Reverse Proxy
Requester - HTTP client plugin for Sublime Text with an expansive feature set, modern UX, elegant syntax and...
Wireshark - Wireshark is a network protocol analyzer for Unix and Windows. It lets you capture and interactively browse the traffic running on a computer network.
API Test Base - API test automation tool that has GUI, supports HTTP stubs (mock services), data driven testing, and API types beyond HTTP, such as relational database, JMS, FTP(s), AMQP, MQTT, etc.
Postman - The Collaboration Platform for API Development