Snarl

Snarl is a notification system that displays the messages sent by your different applications right in the system tray. Made to look like Growl (a similar tool for Mac OS X), Snarl creates a small, semitransparent window where you receive notifications of all kinds. For now, it works better with system notifications. It doesn't have much support from software developers, since there are only a few applications that are compatible with this system. Besides that, you need to install an extension to use it with Firefox, Pidgin, Quintessential Media Player, Songbird, Sunbird, and Thunderbird. Click here to access a page where you can download these extensions.