Google released a simple IM and VoIP application the other day, with additional features on the way in future releases.
The service is simple, but designed in typical Google "elegance". It allows users to IM with other subscribers as well as talk through a VoIP connection. Currently a Gmail user account is required to sign up, and if you don't have one, the service automatically sends you an invitation from the user who introduced you to the service (if they have available invitations). A neat feature automatically loads your Gmail contacts into the IM client.
The service is currently in beta. Read more about Google Talk.
All the best VoIP ing!