The Developer Briefcase is a set of tools to make debugging and analysis of web pages easier for developers.
The Developer Briefcase is also useful for checking mistakes and making sure your pages are as error free as possible.
The functions included in the extension are essentially bookmarklets useful for web developers. However, this way, they are all in one place in an easy to use manner.
NOTE: Once you install this extension, then make sure to refresh any already open pages for the extension JS to be applied to it for it to work on them.
The most full-featured Internet power tool on the market, Opera includes pop-up blocking, tabbed browsing, integrated searches, E-mail, RSS Newsfeeds and IRC chat.