I am using a bookmarklet which can (obviously) be called by the user anywhere:
javascript:(function(){window.open('http://*****.com/***.html?url=
'+encodeURIComponent(window.location.href),'my_page_name',
'left=200,top=200,width=480,height=500,personalbar=0,
toolbar=0,scrollbars=1,resizable=1')})()
How can I make this like a modal window, meaning no ugly browser window borders - should I use jquery or something like in the bookmarklet URL and if so, how?
You could use the approach Firebug Lite uses.
Basically you insert an external JS file into your page when you click on the bookmark.
Just change BASE_URL, PATH_TO_JS and PATH_TO_ICON to what you need it to be.
Also, don't forget the "http://" in the BASE_URL, unless you want to use a relative path.
Your external JS file could contain a script which adds an element to the page which hovers over others. I recommend using the CSS in Twitter Bootstrap to figure out how to make a reliable modal window.
Edit --
To help you out I wrote a small demo. It consists of 2 files:
bookmark.html
bookmark.js
Demo at: http://bijtel.com/stackoverflow/bookmark/