Freeware Xobni integrates into your Outlook installation and shows you more about your e-mails than Outlook can 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.
Unlike many other Outlook add-ons, Xobni seems to add its functionality without dragging down Outlook's performance, or worse, crashing it. It's useful and it doesn't get in the way. There's no reason not to try it.
Still in beta, 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. Future plans for Xobni include integration into Outlook's default search bar and support for other e-mail apps like Yahoo and Gmail, and make the platform available to other vendors--all of which indicate that Xobni definitely wants to be here for the long haul.