Add this button: http://operawiki.info/PowerButtons#kill
It highlights in red elements of the page when you hover them. Clicking, pressing enter or pressing space removes the element and goes back into normal mode. Pressing escape goes back into normal mode.
Click it and Opera will ask if you want to add it and it will list its action. In this case this just uses the opera button "Go" action to execute some javascript (a bookmarklet). The button will then be available under "My Buttons" when you customise any toolbar. You can also just drag the button off the page onto any toolbar. However, if you do this, it won't go into "My Buttons". Thus, if you delete it accidentally later you won't be able to add it unless you go back to that wiki page.
The following pages have a lot of useful buttons. There are also several online button creators that allow you to make your own button with icons/actions/labels easily if you are not comfortable editing .ini files manually.
http://operawiki.info/CustomButtons http://operawiki.info/PowerButtons
Alternatively you can add the same script directly as a bookmarklet. A bookmarklet is a bookmark that contains javascript instead of a url. Bookmarklets work in any browsers that can execute the script properly. In Opera, you can add bookmarkets to the Personal toolbar just like any bookmark, or click them from the Bookmark panel (Ctrl+B) directly. To add a bookmarklet to other toolbars you have to make them into opera buttons. However, the disadvantage of this is that if the script is too long it won't work as an opera button.
Bookmarklet:
javascript:var b=new Array();var c=1;var o=((document.onkeydown==null)||(o==2))?0:1;document.onkeydown=ck;z=document.getElementsByTagName('*');for(i=0;i<z.length;i++);z[i].onmouseover=function();void(z[i].onmouseout=function());}}function ck(e)}}}if(o==1) ck(1);