Google announced its instant messaging and voice program today. Google Talk allows you to talk and send instant messages for free.The user interface minimalist as you would expect from Google, which makes it very efficient and a easy to use.
Google is doing The Right Thing and making an open system that also works with other clients:
In addition to the Google Talk client, there are many other clients out there that provide a great communications experience. We believe users should have choice in which clients they use to connect to the Google Talk service and we want to encourage the developer community to create new and innovative applications that leverage our service. To enable this, Google Talk uses the standard XMPP protocol for authentication, presence, and messaging.
The great news is that they are planning to do the same for the voice functionality. At the moment they use a custom protocol but they are planning to add support for SIP and federate with other voice providers:
We plan to partner with other willing service providers to enable federation of our services. This means that a user on one service can communicate with users on another service without needing to sign up for, or sign in with, each service.