I learned about xobni from a colleague last week, and downloaded it today. Xobni ('inbox" backwards) bills itself as "the Outlook plug-in that saves you time finding email conversations, contacts and attachments" and / or "a more socially aware email environment." I would prefer to describe it as "the free Outlook tool with addictive metrics." Once it started analyzing, showing me, for example, who among my contacts is quickest to respond to me, I was fascinated. Then when it gave me the option to send said contacts their stats, I was hooked.
The design is bright and appealing (maybe a little too appealing - compared to the bland light-blue-and-white Outlook color scheme its orange, black and purple tones make it difficult to look away or ignore the pane) and I found it user-friendly and intuitive. After eight hours I'm thinking of my correspondents in terms of their xobni rank ("Yeah, he's my number three") and trying to figure out imbalances ("Why am I sending her two emails for every one she sends me?"). It interfaces with LinkedIn. It bubbles-up attachments. It threads conversations. It hasn't broken anything on my pc, although the comments on this post about xobni's business model indicate that it may cause problems with FireFox. For my setup, there is no downside and a tremendous upside.
I would rave more, but I've got to go reply to an email from my #1.