Gmail-Users - join
Talk about Gmail, good or bad. This is not a group for invitation requests. Please see http://groups.google.com/group/GiveAndTake for requesting or handing out invitations. Requests for invitations on this group will not be posted.
Medium activity, 16413 members, moderated

GMusers- GMail Beta & Beyond - join
A core group of GMail users who expect to be using GMail through beta and beyond. The list is moderated. There are multiple moderators in order to preserve diversity of opinion and a smooth flow of posts. GMusers is open and globally inclusive. You do not need a GMail account to join.
Low activity, 3235 members, moderated

GTray - join
A place to discuss GTray, a GMail notification tool. Also to share interesting information about GMail technical design and inner-workings and how to create enhancing third-party software to an already great service.
Low activity, 944 members, moderated