Postman - The Collaboration Platform for API Development
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.
Insomnia REST - The most intuitive cross-platform REST API Client ð´
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.
RESTer - A REST client for almost any web service (Firefox and Chrome extension).
Postwoman - An online, open source API request builder