Snarl is the Windows equivalent of OS X's Growl notification application.
The default Windows notifications are terrible. Windows can only display changes one at a time and only for very specific applications. Snarl remedies Windows' poor notifications with their application and API for developers to integrate into their applications.
Snarl is easy to install and is completely open source (free!). After installation, system changes like the clock and network are automatically registered with Snarl to show notifications. There are a ton of options in Snarl's settings to allow the user to change behavior as well as look and feel. You can also download more themes and plugins for the program.
While Snarl supports popular applications like foobar, Pidgin, iTunes, and Spotify, it's complicated to register applications with them. For example, you have to download an extension for foobar to get it to work with Snarl. Growl for Mac, meanwhile, automatically picks up on any new application that supports its notification protocol, which would be nice to see on this Windows version.
Overall, Snarl is a great replacement for the default Windows notifications, but it could use some work making it more user friendly.