Xobni integrates into your Outlook installation and shows you more about your e-mails than Outlook ever could. For each person who sends you e-mail, it shows you who else they communicate with a lot--their de facto social networks--and it finds their phone number from inside their e-mails. It also 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 an admittedly redundant but snappy e-mail search engine built-in.
Unlike many 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, but be warned: it's still in beta, so just because others haven't had problems doesn't mean you'll get away scot-free.
UPDATED: Xobni, is of course, not open-source, although they do have some open-source plug-ins. Apologies for the confusion.