For the past few months, I've been using LookOut for Outlook to tame my ever-enlarging Inboxes. It does a phenomenal job and it's free (http://www.lookoutsoft.com). I don't think I could live without it as I have instant access to all my archives.
But now, I just installed Google's Desktop Search tool http://desktop.google.com/, a tool that essentially "googlizes" your entire computer: files, emails, everything - is now accessible from a Google Search.
They both have effectively done the same job: LookOut has an option to look into My Documents, if you configure it and Google has an option to look at your email.
The thing I really like about Google's search is that it automatically groups my emails. So if I do a search for Customer Contract, and there are 5 different email threads spanning 100 emails - they appear as 5 "found" items but note how many emails are on each one. LookOut , in contrast, lists each email which can make it hard to move through.
Google's search also seems much faster.
Some people note some security options on the Google Desktop search (http://blogs.pcworld.com/staffblog/archives/000264.html) but those can be turned off.
Let me know which one you prefer.