Starting today, Xobni is publicly available for anyone to download after seven months of invite-only beta. I use it since a few months and it changed the way I navigate in Outlook: Now most of the time, when I search a mail, I remember who sent me that mail. I search that person and afterwards I browse the conversations with that person to find the mail.

Xobni: Email organization, search, and navigation for your Outlook inbox via kwout

There are additional information on Xobni in this New York Times Article.