Most apps have to work with web APIs. Paw is a HTTP client with a GUI. It lets you set up projects. You can run and inspect API calls and even let it generate Objective-C/Swift code making those API calls for you, which you can copy to your own codebase. I like to use it as a live document. Making API calls with both valid and invalid parameters and inspecting the results.
It's a wonderful tool to make working with APIs easier.