Chrome Mobile DevTools - Mobile Debugging Tools
Firefox Developer Tools - Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.
HttpWatch - HttpWatch: The ultimate HTTP Sniffer for Chrome, IE and iOS
Google Chrome Developer Tools - Get started with Google Chrome's built-in web developer tools.
Xcode - Xcode is Apple’s powerful integrated development environment for creating great apps for Mac, iPhone, and iPad. Xcode 4 includes the Xcode IDE, instruments, iOS Simulator, and the latest Mac OS X and iOS SDKs.
Code Cola - Code Cola is a chrome extension to editor online pages' style visibility.