I might be late with this answer, but if anybody comes here looking for a simple, light, minimalist, and unobtrusive notification plugin, I've made an open source jQuery notification plugin that can be integrated seamlessly with web apps, called jNotifyOSD. You can see a demo on that link. I've tried to keep the API clean and dead simple to use. Here's an example:
$.notify_osd.create({
'text' : 'Hi!', // notification message
'icon' : 'images/icon.png', // icon path, 48x48
'sticky' : false, // if true, timeout is ignored
'timeout' : 6, // disappears after 6 seconds
'dismissable' : true // can be dismissed manually
});
You can also set global defaults for all future notifications (can be overridden on a per-notification basis):
$.notify_osd.setup({
'icon' : 'images/default.png',
'sticky' : false,
'timeout' : 8
});
It includes a very nice default theme with transparency on hover (meaning that notifications get more and more translucent as the mouse pointer approaches), but the theme can be changed very easily just by putting in a CSS file which specifies styles for some defined classes. I'm working to include more features, so you should keep an eye on the GitHub repository.
UPDATE [13th Dec, 2012]:
It's been some time, but I've finally implemented support for multiple visible notifications using a queue system. So for example:
$.notify_osd.setup({
// ... config ...
'visible_max' : 5 // max 5 notifications visible simultaneously
'spacing' : 30 // spacing between consecutive notifications
});
You can see a demo here. I think the plugin is now flexible enough to handle a good variety of use-cases.