RapidAPI Design by Paw is an API client that offers features such as authentication, testing, and documentation. It has a user-friendly interface and supports multiple programming languages. Postman is another popular API client that provides features like mock servers, API documentation, and collaboration tools. Advanced REST Client is a simple yet powerful tool for testing APIs, while Nightingale is a Windows-focused API tool with features like request inspection and dynamic variables. ReadyAPI (SoapUI Pro) offers advanced features for testing and performance, including assertions and security scans. Runscope is a monitoring platform that also provides features for testing and analytics. Rest-Assured is a Java-based API validation and testing software, while Insomnia is an open-source API client with features like code snippets and collaboration tools. Inso is a CLI tool that accompanies Insomnia, offering features like linting and test generation.