Gears
Green Granite, Gmail works fine with FF 5 &6. But Gmail offline does not, which is well-documented. A key point of Gears is to provide offline Gmail and Google Reader, which is extremely useful when travelling, especially on aircraft.
Google has ended development of Gears in favour of an html5 "solution" which however does not actually exist yet. This is the sort of thing that means Google and the cloud in general is still far from being a serious alternative for business.