Growl is a notification tool that puts little unobtrusive message bubbles on your screen to let you know what's happening in the background. It doesn’t work with many of Apple’s built-in apps, but does work with a number of third-party programs and extensions. These include file transfer programs like Transmit, instant messaging programs like Adium and Yahoo, dashboard widgets like eBay watcher, and many Mac twitter apps. This can be when an upload or download is complete (Transmit), when a new IM message comes in (Adium), or when your plugged-in status on your laptop changes (Unplugged).
An outstanding introduction to the core of the Objective-C language.