A popular XUL overlay is GreaseMonkey which the author showed could be used to add features to web sites such as persistent searches to GMail all using client side script.
Drew Amato submitted a simple patch that makes the searches box remember its expanded/collapsed state. Norman Rasmussen submitted another that makes sure the searches box ends up between the labels and invites boxes. Both of these have been incorporated into the script.
Finally, this is only tangentially related, but Brad (of the LiveJournal fame) has come up with an even cooler use for data: URLs: a shared whiteboard (description here). The whiteboard data is refreshed via a base64 encoded PNG that is returned from the server via a
XMLHttpRequest object. Crazy stuff.