Can someone suggest a good lightbox like popup div?

I'd like it to just display a div that's hidden on the page so it doesn't even need ajax.
Just click a link, the page goes dark and a popup div shows up in the middle of the screen.
It's important that I can scroll the page without loosing the effect.

You're talking about a shield underneath a preference or options box. The shield is also a hidden div. Usually people make it "red" and set it to 50 opacity so that when displayed, the text underneath can be seen but not selected (except in Opera).

One can center a div using styles in a rule for the div which read - marginTop: auto 0px; marginLeft: 0px auto;

'Ajax' so-called works whether a div is hidden or not, and works regardless of whether there is a div.