250 million people access yahoo mail every day. They are building web 2.0 experiences to Yahoo mail.
When email it will show you if the other user is online. If you click that the email window turns into a chat window and let’s you chat with the other person.
If the other person goes offline their name turns into a link that lets you send an email. The body of that email automatically contains the content of the chat.
They are adding social features to email. Kind of cool and applicable to everyday users.