Here's a must I'm trying out (it's free!): xobni. Xobni is the Outlook plug-in that saves you time finding email conversations, contacts and attachments. Here's what it does:
Lightning fast email search.
The email or person you're looking for appears as you type
Hello, threaded conversations.
Email how it was meant to be read, with conversations in context
Rankings, graphs, and statistics detailing how you and your contacts use email
Quick attachment discovery.
Historical attachments available instantly without searching
Navigate your inbox by people.
Discover email's social network - quickly identify a contact's manager, business partner, or assistant
Phone numbers extracted from emails.
Contact details pop into the sidebar - never search for a phone number again
Your personal assistant.
Schedule appointments with a click of a button - Xobni automagically finds open slots in your busy schedule.
Except for the search index duplication (with Windows Search), where I have some doubts, xobni is a must! I'll post my experience here in a week.