Google's Chat service, GTalk, is based on XMPP, the protocol behind Jabber. That's why you can use any Jabber client with GTalk. This has other implications beyond chat clients, however. XMPP has a very capable authentication mechanism built-in to service distributed chat servers, but you can use XMPP authentication for anything. Google has conviniently tied this authentication service to your Google account. That means that you could build an application that let's people log in using their Google account name (what I call GIDs) and password without any prior arrangement with Google. With no fanfare at all, Google has created a universal login for anyone who wants to use it.