The biggest new feature in Opera 11 is extensions. Opera has always had a lot of features out of the box - it has a feed reader, email client, download manager etc. However, with extension support, developers and users can further extend the capabilities of the browser. For a very long time lack of extension support has always been a big hindrance to people using Opera as their primary browser.
Around 200 extensions are already available for Opera 11 right now. You can get the extensions here.
Firefox came up with a solution to managing multiple tabs - they called it Panorama. Tab Stacking is Opera's answer to Panorama - and it simply far better than Panorama.
With Tab Stacking, users can simply drag and drop a tab into a similar tab. The tabs will be merged and hovering over that single tab will reveal all the tabs. It is such a simple and elegant feature that users will be able to use it straight away.
Tab Stacking and Extension are the major features in Opera 11, but there are others as well. Opera 11 has a new address bar which hides the complexities of the URL and gives a visual indicator of the safety of the website.
Another improvement in Opera 11 is the visual mouse gesture. With the visual mouse gesture, when the right mouse button is kept pressed an indicator indicating the various gestures you can do overlays the screen. This means users do not necessarily have to know all the gestures to use it effectively.
Opera 11 is available for Windows, Mac and almost all the major Linux distros. Click on the link below to download.