After release Opera 11 Beta version for some months ago with some new feature, Opera has released Opera 11.01 final version for Windows, Macintosh, and Linux. Opera 11.01 includes lots of security fixes and stability improvements as well as some mouse gesture tweaking. Some new and great feature from Opera 11 are tab stacking and extensions feature.
Tab Stacking is grouping opened tabs. You can drag tab into another tab and create groups of some tabs. It will be more efficient when you open many tabs in your Opera.
Opera 11 allow you to add extensions which you will not find on previous version. With this feature now, Opera 11 is like Firefox and Chrome that has add-ons and extensions features.
You can check another Opera 11 feature at Opera Browser Features
* Images taken from opera.com