David Pogue reviews the latest VOIP (Voice over Internet Protocol) services, which allow you to call anywhere in the United States for as little as $20.55/month (plus your broadband internet connection):.

This development is annoyingly called voice-over-Internet protocol, or VoIP, which means “calls that use the Internet’s wiring instead of the phone company’s.” When you sign up, you get a little box that goes between your existing telephone and your broadband modem (that is, your cable modem or D.S.L. box, a requirement for most of these services).
At that point you can make unlimited local, regional and long-distance calls anywhere in the United States for a fixed fee of $20 to $40 a month (plus the cost of your broadband Internet service, of course). Overseas calls cost about 3 cents a minute. These figures aren’t subject to inflation by a motley assortment of tacked-on fees, either; voice-over-Internet service is exempt from F.C.C. line charges, state 911 surcharges, number-portability service charges and so on.

Save money, switch! I’ve been using www.packet8.net for some time.
Two alternatives beyond the phone interface: ichat | skype