The Opera developers have just announced that Opera 11 will finally get extensions support. The announcement also states that porting extensions from other browsers (they are most probably referring to Chrome) will be very easy and the extensions will either display a new button in the browser UI or will run in the background without any UI element.
I don't know about you, but for me this is very exciting news as the extensions (even though opera offers a lot of functionality which you'll not find in other browsers by default) were the only thing that kept me from using Opera as my default web browser.
Alpha builds of Opera 11 will soon be available for download @http://www.opera.com/browser/next/
For more info, read the announcement.