HttpWatch - HttpWatch: The ultimate HTTP Sniffer for Chrome, IE and iOS
Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
Postman - The Collaboration Platform for API Development
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
Insomnia REST - The most intuitive cross-platform REST API Client ð´
TrueMetrics Analytics Inspector - Analytics Inspector provides an easy way to validate web analytics and marketing integration, as well as offers a great way to quickly validate the instrumentation via the unique and simplified user interface.