The Opera browser has for a long time included an incredibly useful and powerful debugging tool called Dragonfly, that offers functionality similar to the FireBug add-on for Firefox, but comes integrated with the browser, and offers more features. Now it has finally been released in its first (1.0) stable version.
Dragonfly comes with the usual package of tools that one would expect from any browser-based debugger / development tool. Dragonfly has:
One of the best features of Opera Dragonfly is its ability to debug remotely. You can use this feature to not only debug websites running on the local Opera instance, but also those running on Opera for mobile, tablets and other devices.
While Opera itself is closed source, Dragonfly is an open source technology under the Apache 2.0 license, and the protocol for communication between the debugger and browser is likewise open source. This means it is possible for other browsers to support Dragonfly and be debugged using it.
The developers of FireBug have their own plans for something similar. A "Multiprocess Cross-browser Open Web Tools Protocol" called CrossFire that will allow similar functionality. More competition, almost always a good thing.
Since Dragonfly is loaded remotely in the Opera browser, to take advantage of it, all you need to do is install the latest version of the Opera browser installed.
Here is a video by Opera describing Dragonfly: