Skype is a free VoIP client designed along similar lines as the various popular IM clients.
All of the technology needed to have a decent VoIP system is readily available: a sound card, a microphone, and an Internet connection (preferably broadband) are really all you need to use Skype. Like an IM client, you have a buddy list, and you can configure it so that only the people on this list can "call" you.
A Skype call works not unlike a normal phone conversation: just double-click on a buddy on your list, and that person will have a window pop up and hear a sound effect of a telephone ringing, from which they can accept the call.
And that's it; it's just like a phone conversation from here on, until you hit the disconnect button. The sound quality is exceptional, being far better than a normal phone call, so long as the person you're talking to doesn't put their microphone right up to their mouth.
Skype boasts a number of features. All calls are encrypted, preventing casual eavesdropping. A service called "SkypeOut" (which is not free, unlike the rest of the program) allows users to dial normal phone numbers from the system. A normal IM client is included, which is useful if (say) the person you're calling doesn't have a microphone. You can transfer files using the system, too, just as most IM clients allow you to do. It also supports conference calls of more than two people.
All told, Skype is an excellent program. If you find yourself needing to make a number of long-distance calls, convince the person on the other end to install this to save yourself a bunch of money on your phone bill. The Internet, of course, doesn't care about long distances.
It can be found at www.skype.com