Don't know if you realize it, but you have always(?) been able to do this in Opera without resorting to extensions or customizing. If you simply shift-click any link or form buttons, it will open in a new tab (or window, depending on your settings). If the form button you pressed submits the form, the results will of course be submitted to the new tab or window as well. Should you, like me, prefer to stay away from the mouse when typing in a form, you can achieve the same result by hitting shift+enter to submit, instead of just enter by itself. This keyboard shortcut extends to other parts of the interface as well, and will work in elements like search boxes to.