Mockoon is the easiest and quickest way to design and run mock REST APIs.

XCUITest is Apple's native UI testing framework for iOS, tvOS, and watchOS apps, built directly into Xcode

RapidAPI for Mac is a full-featured HTTP client that lets you test and describe the APIs you build or consume. It has a beautiful native macOS interface to compose requests, inspect server responses, generate client code and export API definitions.

Diffblue Cover uses next generation autonomous AI to automate unit testing so that Java development teams can build better applications faster.

NUnit is a unit-testing framework for all .Net languages.

Simulate APIs, integrations, and environments at enterprise scale.