To sell upgrades to an operating system, Apple knows it has to deliver impressive treats to all its users. Otherwise people stick with the OS they have, with its known flaws, predictability and price already paid.
Well, Panther doesn't let anyone down when it comes to new and improved features. The app with the most long-term impact is iChat AV. The full version is included free.
iChat introduces personal audio and video conferencing for the rest of us. This is full-screen video and/or full duplex audio (real-time, two-way voice communication) with a simple user interface. The video works with Apple's new iSight camera or any other Firewire-capable camera.
What makes iChat special is its zero configuration demands, its high-quality results for very little bandwidth, its overall low cost, and the fact that people can now see who they are talking to. Add to this the fact that using iChat over a high bandwidth internet connection is cheaper and has better quality audio than the telephone, and it becomes a no-brainer.
The only thing keeping it from completely taking over the telephone market is that it needs to be supported by some form of chat or instant messaging service.
AOL has such a service, as does .Mac (dot Mac). Both parties need one of these accounts. And if both of you want to chat with video, you will also need DSL or a cable internet connection.