That was fast! Just last week we were telling you about Opera getting extensions support and today, Opera 11 alpha has been released. The extensions work just like in Chromium: an icon is displayed next to the search bar (or no icon at all) and you also get an extensions page from where you can disable or uninstall the extensions (to reach this page you either have to right click any extension and select "Manage Extensions" or press Ctrl + Shift + E).
To try out the new extensions or submit your own, see https://addons.labs.opera.com/
Besides extensions support, Opera 11 alpha also comes with Opera Presto 2.6.37 which brings improved performance and other enhancements.
Download Opera 11 Alpha (includes .deb files) - remember, this is a test version so expect to find bugs!