[theapportioner]

It organizes emails into conversations, so if you and a friend send a dozen reply to's it'll organize the entire conversation for you. Each of the messages in the conversation would have it's own tab that you could click, kinda like how ICQ used to work when you looked at the history of a convo.

Other than that, a few other features like shortcut keys, and in the mailboxes the first part of the message is previewed, next to the subject header.