Many software testers get by with a small toolset but testing an API requires expanding on that. New tools emerge and others go out of fashion every day, but at least we can make a list of what is relevant right now. Here are some suggestions to help your process of choosing an API testing tool.
API Science, a leading API monitoring service, has launched a RAML import capability that allows monitors to be created by importing any RAML definition. RAML is the RESTful API Modeling Language based on YAML, that allows developers to document APIs in a human-readable form.