Xobni (inbox spelled backwards) is a windows-only, freeware application that adds some pretty neat features to Microsoft Outlook. Some of these features are really gee-whiz and some actually seem pretty useful.
It displays some of the sender's contact info, the frequency of their emails, times at which they're sent, etc. It also provides threading for your conversations, which is actually pretty neat too. That is, you can click on a particular conversation and Xobni will assemble the thread in the panel.
Check out the video for a better explanation.
Xobni is in private beta at the moment. I've got 5 invites if anyone is interested in trying it out.