Dovecot supports it, but are there any good, stable IMAP clients that work with it?
(Although I usually kill the default tags that Thunderbird still includes as a hangover from the original Thunderbird release because that way I get useful tag names in the Maildir should I ever need to hack on them directly.)
I use Thunderbird with a cheap fastmail.fm account. You can access your fastmail account from anywhere with their web interface. You can access it from any imap client.
It all works good enough for me. Not fantastic, but good enough.
Oh man. As a longtime Thunderbird user, I have to say, that's a loaded question. :)
Sadly, a full on "here's a hierarchy of tags, just display mail by tag" mail reader doesn't exist to my knowledge outside of GMail itself :(
I love nearly everything about Pine, but I wish that Dovecot's IMAP searching was faster.