Here's a simple but flexible javascript popup function. This effect is known as a chromeless window. This usually means that the window is resized and the browser features (nav buttons, address field, scrollbars, resize handle, and favorites) are hidden or altered.

Javascript Source Code:

<script language="javascript">
<!--
var openwin;
function popupwin(url,myname,w,h,s)
{
settings='height=' h ',width=' w ',scrollbars=' s ',toolbar=no,location=no,status=no,menubar=no, resizable=no,dependent=no'
openwin=window.open(url,myname,settings);
}// -->
</script>

HTML Source Code:

<p>
Click here to
<a href="#" onClick="popupwin('/index.php','Loading','400','400','yes');">
Open Popup Window</a> the pop up.
</p>

Adding it to your site:

First create a variable called "openwin". Next create a function called "popupwin()". This does all the work. To configure the code for your site, just change the values inside the javascript "settings" line and the html link "onClick" values. And that's it.

Click here for a working sample.