I've been using Xobni for the past couple of months.  Xobni ('inbox' spelled backwards) is a plugin for Outlook.  The theory is that email is the ultimate social network.  Once you've installed it, it goes through your outlook history, indexes key words and conversations you've had with your contacts and makes all of this pretty easy to navigate.  For instance you can type in the name of a contact and easily pull up their contact info, find past conversations and see what attachments you've sent to them.  There are also bells and whistles like seeing how popular a contact is with respect to your email activity and even the time of day that you get emails from someone (to optimize your response times).  They finally took it out of beta today and I would recommend it to anyone looking to make Outlook more productive.  My only wish is that they would index archived .pst files as well as your current one.  There have been rumors that Microsoft was going to buy Xobni.  I'm surprised it would have taken them this long.