VOIP has been changing the telephone and communications industry for the past several years. From giant enterprise devlopments to individual PC users, the ability to communicate over IP networks opens up huge possibilities that never before existed.
Skype brought VOIP to the computing masses with a simple client that worked right. Unfortunately the the proprietary system that Skype uses restricts further innovation by 3rd parties.
Gizmo and ineen are two VOIP software packages that use the SIP standard to communicate with anyone using a compliant software package.