Postman is a great tool, like @Hipkiss mentioned.
Also, while this suggestion may not be exactly what you are looking for, I'll mention it anyway. I recently found this app named Quicktype that I think will be very useful for me. It's standout feature is that it can take in a JSON object and derive a JSON Schema or various other type definitions for that object. It can also generate code for validating the object in various languages.