Google is almost ready to roll out a feature in Gmail called "Reply by Chat". It lets you reply to email messages using a chat window (if that person is online) rather than the usual message composer.
What's the use of this? Well, if you have enabled the ability to save your chat sessions, the conversation you have will be saved as part of the actual message thread. So regardless of how you communicated with someone, everything is stored as part of the "conversation".
This is a interesting idea and I think the people who use it will love it. Of course, it won't benefit those who either don't use the chat or use an unsupported language -- but more importantly, it won't annoy them either.