Based on a review I read, I thought I'd give Xobni a try. As described on the Xobni home page, "Xobni is the Outlook plug-in that helps you organize your flooded inbox." Xobni installs a new right-hand pane in Outlook and tracks your contacts communications. When you view an email, prior emails, files transfered and other cc'd contacts display in the Xobni window.
Xobni provides you additional information as well, such as the contacts phone number, the numbers of sent and received emails to the contact and how the contact ranks in comparison to your other email contacts. You can also see a graph of when the contact communicates with you (by hour of the day). And finally, Xobni has a search feature to help you find emails. The Xobni website covers the features in more detail.
The reason I'm uninstalling Xobni as I have not found the benefit in comparison to the pain. Every morning when I start Outlook, it now checks for errors because it did not close properly the last time used. It never finds errors; this started when I installed Xobni. Once that process is complete, an already bloated, slow Outlook takes longer than ever to open, due to the load time (I presume) of Xobni. Don't just take my word for it, you can check view comments from other unhappy Xobni users on the Xobni site -- see the comments for this article.
I wanted to like and use Xobni, but it just wasn't working. Assuming in the future Xobni solves the performance issues, one other things to consider is the value to your email management style. I file emails by subject, not contact, so many of the features were not as beneficial to my style. When I want to go back and find an email correspondence or file, I look under the subject, not the contact who may have sent it to me.
If you are interested in finding other Outlook plug-ins to assist you in managing email, I found a few potential helpful sites: