In case you were looking for thedel.icio.us button for Opera, I've kept that pagefor you here for now:

Posting links to del.icio.us with Opera

To the button, to the sourcecode, to the bookmarklet.

This page was just updated for the new del.icio.us relaunched by AVOS Systems and the new Opera 12 (I'm using Opera Next).

The new del.icio.us button:
post to del.icio.us
(to install the button just drag the link up to your personal bar)

Screenshots

avos.com/new-delicious opened in an Opera window, some text selected.

The "post to del.icio.us" button and the corresponding tooltip are highlighted.

After clicking the button this popup opens. The selected text is highlighted both on the page and in the notes field.

Sourcecode

opera:/button/Go%20to%20page,
%22javascript:
  (function(){
    f='http://www.delicious.com/save?url='
      +encodeURIComponent(window.location.href)
      +'&title='+encodeURIComponent(document.title)
      +'¬es='+encodeURIComponent(''
        +(window.getSelection?window.getSelection():document.getSelection?document.getSelection():document.selection.createRange().text))
      +'&v=6&';
    a=function(){
      if(!window.open(f+'noui=1&jump=doclose','deliciousuiv6','location=1,links=0,scrollbars=0,toolbar=0,width=550,height=585'))
        location.href=f+'jump=yes'
    };
    if(/Firefox/.test(navigator.userAgent)){
      setTimeout(a,0)
    } else{
      a()
    }
  })();
%22,,%22post%20to%20del.icio.us%22,Compose%20mail

Bookmarklet

As the new del.icio.us bookmarklet comes with all the features we missed in the old version, I now only need to link to http://delicious.com/help/bookmarklets

Button for del.icio.us before AVOS relaunch

In case you are using Opera and Mento you may look for Posting links to Mento with Opera.

If you are using Opera and del.icio.us you probably looked for the same small script I looked for some time. Unfortunately I found some buttons at del.icio.us/help/buttons and even more at del.icio.us/help/morebuttons, but all I found there was not exactly what I needed.

I wanted to have something similar to the Firefox extension provided by del.icio.us. More detailed I wanted a button in my Opera, that opens a small pop-up window and fills out the URL field with the current URL, the description field with the current page title and the notes field with some text that maybe currently selected in the document I want to post to del.icio.us. After posting the pop-up should just close itself.

When I read A Firefox Lover's Guide to Opera I stumbled across Del.icio.us Opera Buttons, which looked quite good, but didn't match my requirements.

Then I thought about extending these buttons to get what I wanted to have. I already began to read at Introduction to Range something about the interesting part of my requirements, which is accessing selected text using JavaScript.
But then, when looking for more del.icio.us scripts I found Better posting to del.icio.us and also bookmarking non-URL file locations. That also was not exactly what I was looking for, but it provided the part I needed to extend the other script.

My next step was to merge some parts of all the different scripts together to create my new del.icio.us button:
post to del.icio.us
(to install the button just drag the link up to your personal bar)

Sourcecode

opera:/button/Go%20to%20page,
%22javascript:loc=location.href;
  title=document.title;
  notes=window.getSelection();
  void(
    open('http://del.icio.us/post?v=4;noui=yes;jump=close;url='+encodeURIComponent(loc)+
      ';title='+encodeURIComponent(title)+
      ';notes='+encodeURIComponent(notes),
      'delicious','toolbar=no,width=700,height=400')
  );
%22,,%22post%20to%20del.icio.us%22,Compose%20mail

Screenshots

del.icio.us/help/thirdpartytools opened in an Opera window, "For Opera" selected.

The "post to del.icio.us" button and the corresponding tooltip are highlighted.

After clicking the button this popup opens. The selected text "For Opera" is highlighted both on the page and in the notes field.

Bookmarklet

For everyone who is not using Opera but want to use this small JavaScript bookmarklet as well, or if you just want to test the script, you could take a look at this link (same source as above, but without the Opera specific parts):
post to del.icio.us