Google Talk - standards based
Well, I know nothing about IM standards, but it sounds like Google Talk is "standards based" from this line here in the faq:
Google Talk also supports standards-based communication clients, which means that Gmail users can access the Google Talk service and exchange instant messages using other clients that support the standard XMPP protocol, such as Trillian, GAIM, iChat, Adium, and Psi. This allows users to access the Google Talk service for instant messaging from all major platforms, including OSX, Linux, and Windows.
http://www.google.com/talk/about.html and
http://www.google.com/talk/otherclients.html might be of interest.
Does anyone know anything about the XMPP protocol and does anyone else support it, such as Skype and the Microsoft IM clients?