Brian Russell of turned me on to the Gizmo Project which is a free VoIP competitor to Skype. Gizmo has several big advantages over Skype. The most important is that Gizmo is SIP compliant and will interoperate with other SIP compliant VoIP systems. Gizmo also has free voice mail which is mailed to you as a .wav file. It has a cute mapping feature, a friendly block message, call recording which is great for podcasters and more.
I’ve been very very impressed with the Gizmo quality and reliability so far.