Windows only: Previously mentioned Microsoft Outlook plug-in Xobni (pronounced "zob-nee") is now available to the public for immediate download. Previously in invite-only beta, Xobni adds email analytics, better contact cards, fast search, threaded conversations, and more to your Outlook inbox. The NY Times explains one way Xobni makes your inbox more of a social network of connected contacts:
Xobni recognises that if an executive sends a copy to someone else on each message he or she sends, it might be to an assistant or another colleague. When someone using Xobni searches for that executive in Outlook, the second person is listed as well.
Huh-wha, you ask? Here, have a video demonstration of Xobni in action.
The Xobni beta download is free, and works with Microsoft Outlook in Windows.