If you use Outlook, it probably won't be your favorite application. The search is famously bad, and finding old emails can be a real pain. Xobni, a plug in application for Outlook, aims to make Outlook a pleasure to use.
Once installed, Xobni spends a few moments synchronizing itself with your Outlook, and once it's finished it offers a range of nice features in a collapsible sidebar. The main draw is the Xobni search, which is fast, effective and gives you great results. If you search for a person, emails from and to them will appear as a conversation, and hovering the mouse over a result will show you the first few lines on the message. It's a much better way to jog your memory than the basic results in an Outlook search.
Another cool feature is the 'Email analytics', which gives you a ranking for the contact, and a graph showing detailing your email relationship. There is also Xobni analytics, which gives you really impressive analysis of your emailing in general. If you're interested in time management, this might be really useful tool.
Finding attachments is also easy with Xobni, and if an email has contact details in it, Xobni automatically picks them out for you on a little personal profile panel. It also features Linkedin.com and Facebook integration, so you can easily find more info on the contact.
You can integrate your Yahoo Mail account with Xobni too, which is nice, although so far that's it so you still have to configure Outlook manually for Gmail or Hotmail accounts.
Xobni is still a Beta release, but runs without problems, and is a fantastic Outlook add-on. You'll wonder how you lived without it.