The feature lets users organize their tabs into groups as they see fit. Simply drag and drop a tab onto another and a "stack" of tabs is created, which users can expand by clicking on an adjacent arrow.
The feature will come in handy for those who tend to have a lot of tabs opened at the same time. With tab stacking, users can save tabs for later perusal or organize them into categories, saving space on the tab bar.
Opera already has the most versatile tab management features among desktop browsers. Users can expand the tab bar to see tabs as thumbnails, open a private tab that leaves no browsing information behind and manage closed tabs.
Other features of note in Opera 11 include further support for extensions, HTML5 and visual mouse gestures, as well as an improved auto-update system, a new e-mail panel and the usual array of bug squashes and optimizations.
You can download Opera 11 beta here.