SpamBayes (http://spambayes.sourceforge.net/) uses Bayesian filtering to root out those evil spam messages.
NoSpyMail (http://www.pcworld.com/downloads/file_description/0,fid,23911,00.asp) is a program that attempts to block Webbugs from beaconing back home.
I don't want to have to open a SECOND program to read what is really just more email. I keep Outlook open all the time so reading my RSS feeds there is what I want. IntraVnews 1.1 (http://www.intravnews.com/)is just the tool to do that. It brings RSS feeds into Outlook and lets you decide what folders to put them in.
Those are my three add-ins.