I’ve been using Gizmo to make voice-over-ip calls for many months now, and I’ve been extremely pleased with it. They have clients for Linux, Mac & Windows, and the call quality has been outstanding when both ends have broadband.
I picked up an inexpensive Plantronics headset with attached microphone which makes extended conversations while working at a computer a joy. Gizmo call quality is to POTS call quality as stereo is to clock radio. I highly recommend checking it out.