Growl is one of my favorite Mac utilities. It adds a system-wide notification system to your System Preferences that other applications can plug into for displaying messages. The version 1.1 update adds a "global positioning system" which apparently keeps notifications from overlaping as well as a close button to notifications (on mouseover), a sound option and various bug fixes.
Growl remains a free download.
[via Daring Fireball]
Update: Growl dev Chris Forsythe has a post explaining just what the global positioning system is, and why it's cool.