Chrome Developer Tools is heavily based on the WebKit Web Inspector, a part of the open source WebKit project. It has taken the most popular and useful properties from the Webkit Web Inspector. It comes pre-installed in the Chrome browser. For Chrome, it’s the best general-purpose tool for development and debugging purposes.

Companion.JS is an IE debugger that has detailed JavaScript error reporting, firebug like console, a JavaScript console useful for inspecting JavaScript objects at runtime, and a toolbar icon to open the Companion.JS panel.

Opera's Dragonfly is a fully-featured debugging environment for Opera Presto-based products that contains all the tools you need to make your life easier when developing web sites and applications.

Firebug integrates with Firefox to allow the debugging, editing, and monitoring of any website's CSS, HTML, DOM, and JavaScript. Plus, it provides many other web development tools. It has one of most advanced JavaScript debuggers available for any browser. Firebug is a free and open source software.

Firebug Lite is a web developer tool that provides Firebug-like functionality in all the major browsers for easy debugging and development.

Firecookie is an extension for Firebug that makes possible to view and manage cookies in your browser.

FirePHP enables you to log onto your Firebug Console using a simple PHP method call. FirePHP is ideally suited for AJAX development where clean JSON or XML responses are required.

a Firebug extension for jQuery development. jQuery expressions are intelligently presented in Firebug Console and DOM inspector.

Flashbug is a firebug extension for Flash in Firefox. It provides extensive Flash debugging add-ons (SWF resources, AMF data, shared objects, traces, policy log).

IE Developer Toolbar, or IE DevBar, is an add-on for Internet Explorer 6 and Internet Explorer 7 that aims to aid in design and debugging of web pages. Internet Explorer 8 has the IE Developer Toolbar built-in, instead of a separate product, which is known as Developer Tools rather than Developer Toolbar.

IE8 and IE9 include tools that web developers need to efficiently debug their sites directly in Internet Explorer. Developers can debug a site's HTML, CSS, and Microsoft® JavaScript from within Internet Explorer 8 (with developer tools), rather than switch between Internet Explorer and a separate development environment.

A must-have simple extension for any designer. It lets you draw a ruler across any webpage to check the width, height, or alignment of page elements in pixels.

The Microsoft Script Debugger is a debugging environment that extends any Microsoft ActiveX® Scripting host application—for example, Microsoft Internet Explorer (IE) or Microsoft Internet Information Server (IIS).

A simple measuring tool for web designers and developers using Opera. It lets you draw a ruler across any webpage to check the width, height or alignment of page elements in pixels.

Pendule is an extended web developer tool for Chrome. It replicates some of the Firebug functionalities and is especially useful in its ability to switch off images and CSS. So this is another must-have tool for any web designer using Chrome.

Safari includes a powerful set of tools that make it easy to modify, debug, and optimize a website for peak performance and compatibility. To access these tools, enable the Develop menu in Safari's advanced preferences.

The Web Developer extension adds various web developer tools to a browser. The extension is available for Firefox and Chrome, and will run on any platform that these browsers support, including Windows, Mac OS X and Linux.

YSlow analyzes web page performance by examining all the components on the page, including components dynamically created by using JavaScript. It measures the page's performance and offers suggestions for improvement.