SoapUI
SoapUI is a widely used, open-source cross-platform API testing tool for SOAP and REST APIs. It provides a user-friendly graphical interface for functional, performance, and security testing. With SoapUI, you can easily create and run complex test scenarios, perform data-driven testing, and generate comprehensive reports.
Karate DSL
https://karatelabs.github.io/karate/
Karate is an open-source API testing framework that combines API test-automation, mocks, and performance-testing into a single, unified framework. Its easy-to-read, language-neutral syntax is based on Behavior-Driven Development (BDD), making it accessible to both developers and testers. It is particularly known for its ability to write tests without needing to write Java code.
Rest Assured
Rest Assured is a powerful, open-source Java library specifically designed for testing RESTful APIs. It provides a simple and fluent DSL for making HTTP requests and validating responses. This tool is ideal for developers who want to write API tests directly in Java, as it integrates seamlessly with popular frameworks like JUnit and TestNG.
Apache JMeter
While primarily known as a performance testing tool, Apache JMeter is also an excellent open-source choice for functional API testing. It supports a variety of protocols, including HTTP and HTTPS, and can be used to test both REST and SOAP APIs. JMeter's GUI allows for easy test plan creation, and it offers robust reporting capabilities.
Insomnia
Insomnia is a collaborative and user-friendly open-source API client for REST, GraphQL, and gRPC APIs. It offers a clean and modern interface for creating requests, inspecting responses, and managing environments. Insomnia is highly popular for its intuitive design and powerful features for API debugging and development.
Hoppscotch
Hoppscotch (formerly Postwoman) is a web-based, open-source API development suite. It provides a lightweight and fast alternative to desktop clients. You can use it directly in your browser to test REST, GraphQL, and WebSocket APIs. Its minimalistic interface and real-time synchronization make it a great tool for quick API testing and collaboration.