I am using Sparrow (Mac). Funny how GMail started as an the best webapp, and now I don't even remember when I last used it in "pure" form. Doesn't really help the "webapps will rule everything" message.
I've been waiting years for something like this. In the meantime, does anyone have any suggestions of third-party desktop applications that sync with gmail?
What is it that you want that a desktop mail client like Outlook or Thunderbird doesn't give you?
They don't preserve the correct labelling. Google rolled their own IMAP implementation to support putting a single email into multiple folders without copying the email. Thunderbird will take a copy of the email if it has more than one label.