I use Firetalk and Dialpad. You can talk to anyone anywhere in the world, as long as they are properly equipped. A control panel on your screen lets you compile a directory of users and indicates when they're online. To connect you click on a button beside a name. If your target isn't available you can leave a voice message.
Calls are free, since the company hopes to make money by selling advertising.
Oddly, the most popular feature on Firetalk seems to be forums. There seem to be lots of participants from the Middle East. Most of my friends in America don't have Firetalk, and some don't have computers. The same call through Dialpad comes to 25 cents, factoring in my monthly Internet subscription rate. Not that there are no problems.