Opera announced on Thursday that the eleventh major version of its eponymous Web browser is now available for download. Opera 11 includes performance and standards compliance improvements in its Presto rendering engine, but the main feature updates include a new extension system and "tab stacking" for easier tab management.

Extensions have long been a way to add new or convenient functionality to the open source Firefox browser from Mozilla as well as Microsoft's Internet Explorer, but 2010 has been the year that extensions have spread to other browsers. Chrome got extensions early in the year, and Apple added them to Safari this past summer. Though Opera has long maintained that its browser included most of the functionality of popular extensions built in, the company saw fit to add an extension system in version 11.

"We have always worked hard to introduce new and bold ideas in web browsing," said Opera co-founder Jon von Tetzchner in a statement. "But, sometimes we want to take an idea and improve upon it. Opera 11 adds a layer of polish to features people have known and loved for more than a decade, while introducing extensions."

If you can't beat 'em, join 'em.

Opera's extensions are built using HTML5, CSS, and JavaScript—the same standards used for Mozilla's JetPack as well as Chrome and Safari extensions—and are based on the W3C Widget specification. Opera noted that throughout the beta period for Opera 11, over 2 million extensions were downloaded. These include popular extensions for password management and automatic translation of foreign-language webpages.

Opera 11 also introduces a new tab management system called tab stacking. While tabbed browsing makes it easier to keep a large number of websites open in one window, having more than a dozen makes it difficult to easily switch between them and keep them organized. In Opera, tabs can be collected in a stack, and collapsed to one visual tab. The stack can later be expanded to switch among a group.

The feature aims to solve the same tab management issues as Firefox's Panorama does, but in our view offers a less complex solution that may be easier to use in practice. It may be easier to understand tab stacking by seeing it in action:

In addition to extensions and tab stacking, Opera 11 also adds a visual guide for various mouse and trackpad gestures that can be used for navigation. And, the address bar has been updated to give a clear visual indication of the safety level of a particular website, noting if there are any potential security issues. And finally, Opera notes that the new version's binary is 30 percent smaller than Opera 10, and can improve performance by as much as 30 percent as well.

Opera 11 can be downloaded for Windows, Mac OS X, and Linux from Opera's website.