Google today launched Google Talk, an instant-messaging and VoIP (voice calling) client, but at the moment it's not even in Google's own top 10 results for that phrase, there being several websites also called Google Talk already (even googletalk.com is taken by somebody else, who at least is being nice and pointing all the traffic at a charity's website). So I thought I'd link to them and help them out.
At the moment the client is a bit basic, not a patch on Trillian Pro -- but free, unlike Trillian. The technolgoy behind it is Jabber, which has the capability to connect to other types of IM networks, so Google also has plans to allow communication with MSN, AIM and the other networks. Until that happens, though, there's no real killer feature to make you want to switch, especially if all your friends use MSN. It'll be interesting to see where they go with this one, but at the moment it's a "wait and see" thing.
More discussion on the Google forums.