IAlert sends you alerts when particular events occur in other apps and lets you interact with them. For instance, when new mail arrive, iAlert can notify you and let you interact with Mail.app. Or it can monitor a website and let you know the page has changed. My favorite is being alerted when a new buddy signs in or out of iChat. I don't have to listen for iChat's login/logout sound and manually check the list to see who came or went.
iAlert comes with plugins for Mail, iChat, iTunes, and Safari. You can also build your own alerts. (See the included instruction manual.)
Note: Growl is a lot like iAlert, but I like iAlert more. iAlert was easier to set up and use, though Growl can be powerful too.