Google has released Google Talk (Beta) - http://www.google.com/talk/

I am very excited to see that Google has released their communications platform on open Internet standards.  Currently Google Talk is based on Jabber / XMPP, an XML based messaging protocol.  According to their website, support for SIP will be available in the near future as well.

Open standards mean a lot of new possibilities for software developers interested in integrating IM and VOIP capabilities.