I just came across two Growl-like alert plugins that should be of interest to HTML prototypers. Growl is a Mac OS X application that functions like a service that other applications can use to show unobtrusive messages in the monitor when things happen on the system.
These javascript plugins work for the Moo Tools or JQuery libraries to provide small in-page layers in a corner or center of the page to do such things as show AJAX alerts for new content updates, e.g. show when new articles posted as iminlikewithyou does, or show changes in page state when someone is editing a wiki page perhaps.
The screenshot above shows the Window Growl plugin with 2 different types of messages in view.