Growl – OS X Notifications
Growl is a free OS X utility that provides application notifications.
Notifications display new information from your applications without having to switch to that application.
For example you can setup an Ebay notification using Growl that will alert you when an auction is ending. A widget like (and highly customizable) popup will display info on that auction when the time comes.
Its very similar to the push notifications that are being implemented on the Palm Pre and iPhone 3.0.
The setup takes a little work up front but can save you some time in the end.