From Ajax Patterns
How can you get the user's attention while they are working in another window.
37Signals' Campfirenow chat app pumps out a Flash-based "earcon" (quick sound) each time a new message appears. This has alll the advantages and disadvantages associated with the audio medium; it will be a good cue if the user is performing typical work or moving around, but a bad cue if the user is listening to something else or has turned volume down, e.g. in a meeting.