GoogleRumors has announced the successful launch of Google Talk. The following quote from the Google Talk page clearly describes the application in as few words as possible.

They say talk is cheap. Google thinks it should be free. Google Talk enables you to call or send instant messages to your friends for free–anytime, anywhere in the world.

The Google Talk client is currently only available for Windows. But, since the Google Talk network is Jabber-based, it can be accessed from a variety of third-party clients, including Adium and iChat (included with MacOS X).