Growl for Windows!

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

An institutional/enterprise-scale distributed real-time messaging and notification system. Zephyr allows users to send messages to other users or to groups ofusers. Users can view incoming Zephyr messages as windowgrams(transient X windows) or as text on a terminal. Zephyr can optionallymake use of the Kerberos version 4 security library or the Hesiodservice name resolution library.

Notifu, a free open source pop-up balloon utilityNotifu is a tool that displays a yellow pop-up balloon in the system notification area, better known as the tray. It is free and open source, give it a try! Notifu uses the same API that Windows uses to show pop-up balloons. You see them when you add a device, such as a wireless network or a USB drive, or when certain system events occur, say, every second Tuesday of the month...Notifu displays a message. So in its simplest form, the message is the only required arguments. Use the /m switch »

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.

Notica lets you send browser notifications from your terminal to your desktop or phone. No installation or registration is required.

Toasty lets you receive push notifications on your Windows Phone 7 device while you are on the go. Some of the key features are:

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