Here is a screencast showing off two modules that I have built on top of the Popups API module. They are Popups: Add & Reference, which allows the Node Reference widget to create new nodes, and Popups: Subedit, which allows semi-in-place editing of just parts of a node.
This demo starts with a fresh Drupal 6 install, with the Popups modules, CCK, Node Reference and Admin Menu enabled.
View it full size. You might need to turn up your volume, the sound is a little low.