I'm surprised this hasn't been mentioned yet (pardon me if it has).
Last week google added a new link to Gmail... Chat. Yesterday afternoon however, I login and see a pop-up in the lower right hand corner "Well Hello" it says. It's a guy on my gtalk buddy list... IMing me... through GMail.
On the left side of the screen under the Inbox links is now my G-Talk buddy list and anyone else I've emailed. Hovering over these names lets me email them or invite them to chat.
When chatting, there is the option to "Go Off The Record" which stops logging the IM on BOTH ends, not just yours. You can set whether or not to have your chats logged by default as well.
Very cool google, very cool.