This website accompanies a talk/workshop (here's the slide deck) that discusses and compares various API testing tools and best-practices. I've re-created the same API test suite in several different tools to make it easy to compare and contrast them in terms of features, ease-of-use, syntax, etc.
This is the REST API that we're testing. It's a fun list of technology-themed superheroes, sidekicks, and villains.
Mocha and Chai are JavaScript libraries that make it easy to write complex API tests using an intuitive fluent syntax that's easy to learn.
Restlet is a free Chrome extension that's so easy to use that anybody can build API tests.
Postman is a free app that makes it easy to build, test, and document your APIs. It has powerful API testing features that strike a good balance between power and ease-of-use.
Stoplight is a powerful API development tool that's based on OpenAPI (fka Swagger). Its tight integration with OpenAPI enables powerful features like automatic contract verification and code-coverage analysis.