Softphones and IM Clients (Score 1)

iptel.org has a pretty comprehensive list of the softphones that are currently available. A number of IM clients (Yahoo, AIM, MSN) also support voice chat, but some of them had trouble going through NAT the last time I checked. The IM clients also require about two or three clicks before you can start talking to each other, so they might not be the easiest solution for intercom usage.