Stitcho is basically "Growl for the Web", but NOT in the sense of being another Javascript-to-Growl bridge (a few of those exist out there, protoGrowl, Growl mootools, etc.)

Rather, Stitcho lets you send notifications from server side code that are delivered to your users via the Stitcho widget. The Stitcho server sits in the middle to route and deliver the notifications.

Also, Stitcho provides widgets for Mac and Windows. (The Mac version can use Growl if it's available.)

Does that answer your question? If not, please clarify.