Growl: global notification system for Mac OS X
Growl is a global notification system for Mac OS X. If you come a windows background, you are used to little balloons popping up all the time, and knowing that there are ten thousand places to check to find the one checkbox you need to turn off the most annoying ones.
Growl provides APIs for developer to provide notification of actions to users. Users, in turn, get one place in the System Preferences to enable and disable these notifications. Growl also supports a few skins and options (like sound effects) for notifications.
The system is only on version 0.5 at time of writing so installation is a bit tedious, but that is largely because you will need to download and install plugins for various apps to use Growl. All-in-all it is a very good idea, and if I still used Windows, I would demand MS do something similar, seeing how they are the worst offenders. I think this just underscores one of the Mac’s guiding principals — your computer is your tool to do your work.