Email clients

I downloaded Firefox 4 here at work to give a spin to. I'm somewhat browser agnostic in general. Unless there is some feature I can't do without, I am fine with most of the leaders. (I still find IE annoying, but Chrome, Firefox, and Opera all are more or less fine for me.)

It does remind me that I want to find an email client, though. I know everyone just uses gmail on the web, and that's fine, but I'm not always fond of the gmail interface and how it works. As I have switched computers too often, I prefer IMAP to POP - although I don't know if that's actually smart since it means I never have a copy of the emails myself anywhere.

The real thing with clients, though, is that I would really like emails to be tagged instead of have folders. More and more I find I would prefer if I could tag them in multiple ways. Gmail obviously does let you do that with labels. Opera seems to do that, but the client isn't stand alone and has always seemed a bit odd. (It also seems to not work with gmail labels well, but then that seems to be a general problem with gmail.)

So does anyone have an email client to recommend? (Or an email management strategy that would make current clients work?)