noty - jQuery Notification Plugin
Documentation and examples are here http://needim.github.com/noty/.
noty is a jQuery plugin that makes it easy to create alert, success, error and confirmation messages as an alternative the standard alert dialog. Each notification is added to a queue.
The notifications can be positioned at the top, topCenter (like GMail), bottom, centre, top left or top right.
There are lots of other options in the API to customise the text, animation, speed, buttons and much more.
It also has various callbacks for the buttons, opening closing the notifications and queue control.
new: Theme support
More information is here http://needim.github.com/noty/.