Just after Christmas, I got onto an Outlook plug-in called Xobni ("inbox" spelt backwards) (via Lifehacker).
Xobni adds a new sidebar in the main Outlook window which provides a wide variety of relevant information on the sender of the currently-selected e-mail. This information includes the number of e-mails you've received from that contact, contact details, who else they've CC'd or been CC'd on in all e-mails indexed by Xobni, quick links to their most recent e-mails, and a list of files sent to or from that contact.
I've used Xobni for only about a week and a half as I've been on leave - but already love the functionality given that I use Outlook all day. I appreciate little features like transition animations and neat, informative graphics which make for a very polished interface. The Xobni search is a step up from normal searching too (but too complicated to describe here).
Xobni is currently in beta and can be signed up for at http://www.xobni.com/, although you might have to wait to be able to download it.
Tags: xobni, outlook, windows, software, beta