Apple includes the FaceTime app with Macs running OS X 10.7 or later. OS X 10.6 users can also purchase FaceTime from the Mac App Store for 99 cents.
Recently Apple made infrastructure changes that broke FaceTime for OS X 10.6. This has been resolved with a new FaceTime 1.0.5 update. If you are having problems with FaceTime on a Mac running Snow Leopard, download the latest update from the Mac App Store.