Here some example code and resource-urls for all webmasters who are too lazy to write own code or simply can't write the code due to missing knowlege. I didn't write all code myself, so mind the copyright notices.

1) Pop-under with session-protection
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<script>

//Pop-under window- By JavaScript Kit
//Credit notice must stay intact for use
//Visit http://javascriptkit.com for this script

//specify page to pop-under
var popunder="http://yoursite.com/popit/popit.php"

//specify popunder window features
//set 1 to enable a particular feature, 0 to disable
var winfeatures="width=800,height=510,scrollbars=1,resizable=1,toolbar=1,location=1,menubar=1,status=1,directories=0"

//Pop-under only once per browser session? (0=no, 1=yes)
//Specifying 0 will cause popunder to load every time page is loaded
var once_per_session=0

///No editing beyond here required/////

function get_cookie(Name) {
  var search = Name + "="
  var returnvalue = "";
  if (document.cookie.length > 0) {
    offset = document.cookie.indexOf(search)
    if (offset != -1) { // if cookie exists
      offset += search.length
      // set index of beginning of value
      end = document.cookie.indexOf(";", offset);
      // set index of end of cookie value
      if (end == -1)
         end = document.cookie.length;
      returnvalue=unescape(document.cookie.substring(offset, end))
      }
   }
  return returnvalue;
}

function loadornot(){
if (get_cookie('popunder')==''){
loadpopunder()
document.cookie="popunder=yes"
}
}

function loadpopunder(){
win2=window.open(popunder,"",winfeatures)
win2.blur()
window.focus()
}

if (once_per_session==0)
loadpopunder()
else
loadornot()

</script>
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You need to change the 'yoursite.com/popit/popit.php' to make it point to the location of your copy of popit. Then add the whole <script />-section beweent the <head> tags of your website.

2) Standart-codes
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Have a look at http://javascript.about.com/library/tools/blpopwindow_maker.htm . It seems to be a pretty nice code-generator.

3) Code-generators
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http://www.hotscripts.com/cgi-bin/dload.cgi?ID=10413 seems also to be pretty usefull.

Thats it for now, but you can submit more ideas to me if you want. Your copyright notices will be kept and you'll be mentioned as author (of course!).