Surge for Mac - Advanced Web Debugging Proxy for Mac & iOS
Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
Weer - A HTTP protocol debugger with Chrome DevTools frontend interface
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
HTTP Headers - HTTP Headers allows you to quickly see the HTTP header information for the current URL.
Code Cola - Code Cola is a chrome extension to editor online pages' style visibility.