Seek - Thunderbird Email Plug-In for faceted search.

I'm a big fan of Thunderbird as an alternative to Outlook. It's not
perfect, but it works very well on both my Windows laptop and Linux
PC. It also has the advantage of (like Firefox) being open to plug-in
and add ons. That means, if there is a functionality you need, there
is a pretty good chance someone else has needed the same.
I get a lot of emails, and sometimes need to find old emails. The Seek
plug-in adds a level of functionality to Thunderbird that is a hybrid
between filtering and search, they call faceted browsing. I installed it
a few days ago, and have been using it daily. There is an excellent
video and download instructions on this site:
My only complaints - It takes up a lot of screen space. Not a big deal since you can toggle it on and off, but if you do, it re-indexes the folder each time, which is slow. It'd be nice if it indexed in the background, but the interface toggled on and off.

http://simile.mit.edu/seek/