Notica!

Snarl is a notification system for Windows. It provides a standard, simple way for application to notify you, the end user, of important events.

Growl for Windows is a Windows-compatible version of Growl , a notification system for Mac OS X.

Canonicals on-screen-display notification agent, implementing the freedesktop.org Desktop Notifications Specification with semi-transparent click-through bubbles.

Growl forked prior to going commercial, working with OSX 10.7 lion

Notifo is a mobile notifications platform empowering developers to build their own services with a scalable and reliable notifications API. The Notifo API is a simple and affordable way to add notifications to any web app or project.