What if you want to avoid all of the complexities of multimedia conferencing and just have an old-fashioned voice chat? Mac P2P clients can offer this option as well.
If you're primarily interested in talk, one of the more popular VoIP options comes from up-and-coming P2P telephony software startup Skype. Created by the inventors of Kazaa, Skype is a wildly popular PC-phone client that has been downloaded more than 104 million times. Now Mac aficionados (along with Pocket PC and Linux users) can join the party. To use the free version of Skype, you must have a sound card, speakers, and a microphone—and you can only speak to other PCs.
For more flexibility, consider Buzzfon p2p 01, which offers free PC-to-PC, PC-to-phone, and even phone-to-phone conversations. Bear in mind, however, that Buzzfon is shareware, and early-release shareware at that, so it won't have the level of support that a well-funded company like Skype can offer.