For better or worse, I'm an Outlook user. I say that because I like having all my mail, contacts, appointments, memos, and the like under one software roof, but find Outlook itself to be bloated, overwrought pig of a program. Fortunately, I've found a tool that makes it a lot easier to live with: Xobni.
Xobni does just a few things, but does them well. First, it indexes all my e-mail to allow for fast, dynamic searching (results appear as I type). Even better, if I click on a message in my inbox, Xobni shows me a complete profile for the sender: contact information (including phone numbers extracted from e-mail), a list of the e-mail "conversations" we've had, a list of files we've exchanged, and, optionally, his or her details on social-networking sites like Facebook and LinkedIn.
You can even check out statistics for each sender, like the number of messages sent and received and a bar chart showing messages sent by hour. I don't find this information particularly useful, but it's kind of interesting.
It's the fast and easy searching that makes Xobni a must-have tool, especially for Outlook 2003 users (Outlook 2007 does have decent mail-search capabilities, but none of Xobni's other amenities). Incredibly, Xobni is free, at least while it's in beta (which it has been for quite a while). Don't tell the company I said so, but I'd gladly pay a few bucks for it.