I've been using Thunderbird, the email client by those fine folks at Mozilla, for a couple of months now. They finally released the 1.0 version (I've been using the 0.8 release for too long) and one of the coolest additions is saved searches.
I used to have all these goofy filters set up that I'd have to remember to run in order to store email by person after I'd read the email. I wanted them in my inbox first where I'd read them and then I'd transfer them to the appropriate folder. Invariably, I'd forget to run it and delete the email. Of course, that was the email I needed later on. Now I just set up a saved search for the person whose email I want to find quickly. After I'm done reading it, I just delete it and not worry about running filters. Of course, now I have to be careful about what I delete from my trash folder though. But that's another battle.