Tab stacking
Tab Stacking is a feature in Opera which enables tabs to be stacked on top of one another. When hovered over, a tab stack will display a list of tabs in that stack. This feature is convenient for people who open over 20 tabs, because it helps organize them. To add a tab to a stack one must drag a tab into the stack. To create a stack a user drags a tab on top of another tab. In order to remove a tab from a stack, one simply hovers over the stack, selects the tab he wants to remove and drags it to the tab bar next to the stack. While this is one of Opera's best features, it is also little known. Only Firefox offers a similar feature called tab grouping. However, tab grouping is not as simple or smooth as stacking. 

Stacking does have some disadvantages. For instance, one cannot close a whole stack at once without closing the entire window. Also, tab stacking does not have tab cycling abilities, which work only without stacks. Another disadvantage is that it is impossible to disable stacking, which causes accidental stacks. Those are very annoying. Despite these drawbacks, when you get used to stacking, you can not live without it.