Google Talk is Google's entrance into the InstantMessaging world, with a client (currently Windows only) that uses XMPP (also known as Jabber) and SIP.

The great thing about Google Talk is that they actively tell you that you can connect to their network using OpenSource software on other platforms, even though they don't have a ClosedSource Google Talk client for your platform.

As of 17/01/2006, Google Talk now supports s2s, allowing users of other Jabber servers to talk with Google Talk users.