Gmail Offline Rated 4 out of 5 stars
It's the perfect extension for Gmail.
It does have a glitch with Gmail Offline and HTTPS enabled.
A workaround for Gmail Manager showing 0 unread messages when Gmail Offline is enabled is available at http://blog.andreineculau.com/2009/07/fix-for-gmail-manager-0-5-7-2/
I would have given it 4.5 if possible.