What are some software add-on's for Microsoft Outlook?

Two software packages I really like for Microsoft Outlook are Lookout and Attensa. Lookout is a search toolbar that quickly indexes your email messages, attachments, folders, and other Outlook files. Because it pre-indexes all files, search results are returned in seconds. Lookout is a free download for Outlook 2000 and later.
http://www.lookoutsoft.com/Lookout/download.html

Attensa is a simple add-on which delivers RSS feeds into simple, easy email messages. Attensa is a RSS reader that adds on to Outlook. Attensa is $30 and is available at:
http://www.attensa.com/

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Adding to the above, there are a number of Outlook plug-ins that can be very helpful so I hope that Jarett doesn’t mind me adding to this entry. If so, we can always break this into multiple entries. I believe that Lookout was purchased by Microsoft and is now incorporated into the Windows Desktop Search.

Some other add-ons include:

Clear Context (free Personal Edition) — adds some features to better organize and prioritize emails by project.
http://www.clearcontext.com

Neo ($35) — a different interface that shows you many, many views of your messages and indexes them for faster searching
http://www.caelo.com/index.php

SpamBayes (free) — a Bayesian anti-spam filter
http://spambayes.sourceforge.net/