iChat buddy alerts
In iChat and Messages, you can turn on a whole bunch of alerts for individual buddies. Want to hear a custom sound when a certain person returns from away, or would you like the Dock icon to bounce repeatedly when that one coworker sends a file? Perhaps you want OS X to speak the alert to you, but you only want it to do this once, not every time the particular event happens?

Just select a contact and hit ⌘-i, or right-click and choose “Show Info”, then go to the Alerts tab. Be sure to hit the menu at the top to see all the different kinds of events at your disposal.