Freeware Xobni integrates into your Outlook installation and shows you more about your e-mails than Outlook can by itself. For each person who sends you e-mail, it shows you who else they frequently communicate with--their de facto social networks--and it also finds their phone number from inside their e-mails. It shows you all conversation threads you've participated in with the person, and all the attachments they've sent you. You can drill into message threads, and it has a snappy but redundant e-mail search engine built-in.
Xobni the app runs on Xobni the platform, which has hooks deep into Outlook. The platform is what enables Xobni to graft a viewing pane into Outlook, something other plug-ins can't do, and it's the platform that gives Xobni access to all the message data that it uses without bogging down Outlook itself. Unlike many other Outlook add-ons, Xobni seems to add its functionality without dragging down Outlook's performance, or worse, crashing it. The interface doesn't get in the way, and in fact makes communication easier by searching Yahoo mail from Outlook, as well as pulling in your contact info from LinkedIn, Facebook, Skype, and others for searching. There's no reason not to try it.
Future plans for Xobni include integration into Outlook's default search bar and support for other e-mail apps Gmail, and making the platform available to other vendors--all of which indicate that Xobni definitely wants to be here for the long haul.