AT&T has been the leading provider fixed telephones in the USA. So, it's about time they got into the VoIP market. Since they offer broadband internet and cable television, AT&T can usually offer a discounted rate for their "bundle" of services. However, like most bundled services, the introductory is only good for so long. Make sure you check AT&T's local offering when you decide to try.
They've got a range of residential-related VoIP services. The one featured to the left is their "premium plan": unlimited local and long distance in the United States. As you can see, they leave something to be desired, when compared with some other, strictly VoIP providers.
With that being said, look into bundled services with AT&T. The discount there may be worth it, especially with other products. The All Distance Online plan is expensive, but other unlimited local services are available. But that's why you compare VoIP services, right?