Postman is an excellent tool for designing and testing RESTful API services. Postman offers a free version with enough features to make it an invaluable tool for mobile developers who need to test RESTful services.
Charles Proxy is a user-friendly way to capture HTTP/HTTPS traffic between your Mac and remote servers. Charles can also capture traffic to and from the iOS simulator as well as physical devices connected to your Mac via USB.
Wireshark is a powerful network protocol analyzer that gives you a highly detailed view of data traveling across your local network.