Gmail Priority Inbox
http://mail.google.com/mail/help/priority-inbox.html

Get through your email faster … Email is great, except when there’s too much of it. Priority Inbox automatically identifies your important email and separates it out from everything else, so you can focus on what really matters. Features Include: a) Automatic sorting – Gmail uses a variety of signals to identify important email, including which messages you open and which you reply to; b) Sections keep you organized – Incoming email gets separated into sections: important and unread, starred, and everything else. Don’t like these? Customize them; and c) Predictions improve over time – Over time, Priority Inbox gets better at predicting what’s important to you and you can even help train it. This has been added to the tools section of Research Resources Subject Tracer™ Information Blog.