Check out Xobni - a fantastic piece of add-on software for Outlook. It allows you to track all your emails by thread, gives you stats on the sender, and tracks all files sent to or recieved by a particular person.  If your inbox is full and you can never find the one email you're looking for, Xobni will make your life easier!

Now there's a new updated version of Xobni - check it out here: 

http://www.xobni.com/download