One of the things that I love LOVE about the opera browser is it's ability to undo and open closed tabs. I hate it when I accidentally close a tab on Firefox/Chrome/IE and notice that I've just completely lost where I was browsing. In Opera, all you have to do is press undo when you're not in some kinda text box, and it'll bring back to all previously closed tabs.

Now I realize that this might screw up with an individual's privacy; after all, all you have to do is press undo on an opera window that hasn't been completely closed and you'll go back to all the previously visited websites in that instance.

But I really find it useful in opera.