jQuery Impromptu is an extention to help provide a more pleasant way to spontaneously prompt a user for input. More or less this is a great replacement for an alert, prompt, and confirm. Not only does it replace these but it also allows for creating forms within these controls. This is not intended to be a modal replacement, just a quick tool to prompt user input in a fashionable way.
via jQuery Impromptu
It is a lightbox like popup for jQuery