Xobni, inbox backwards is described by Bill Gates as "brining social networking into Outlook".
Xobni is a small addon to Outlook which allows users to view statistics and other useful information about emails recived, such as a graph of what time someone sends email so you know when they are most accessable, extracting contact numbers form emails and creating a list of common contacts.
Look at Ben Nunneys blog to see more about Xobni.
After the initial launch of Xobni it looks to have gotten Microsoft a little excited and now they seem to be opening talks with Xobni to look at buying it. The initial bid of $20 Million has been rejected by the company, but it looks like Microsoft will be bringing a bigger deal to the table soon.
More information on Xobni, visit their blog.