Visual Inspector!

Firebug integrates with Firefox to put a wealth of web development tools at your fingertips while you browse. You can edit, debug, and monitor CSS, HTML, and JavaScript live in any web page.

The Google Chrome Developer Tools provide an integrated environment for debugging, optimizing, and understanding a web application, or website, running in Google Chrome. The developer tools are developed partly through the WebKit open source project, where the tools are called Web Inspector.

Examine, edit, and debug HTML, CSS, and JavaScript on the desktop and on mobile.

Opera Dragonfly is a feature of Opera web browser: it is a full-featured development and debugging tool integrated into Opera.