There are many tools that helps you write your emails offline and send or sync them when you are online. These applications are mostly desktop applications. Google availed this ‘offline’ functionality in browser with the help of Google Gears but for the sake of HTML5 and the improving technologies, they have now introduced this feature as Google Chrome extension.
I am sure that sooner or later Google will make it available to all HTML5 supporting browser. Currently this only available as Chrome extension. You can install this Offline GMail Chrome extension and experience it yourself.
Just like any other application in the world, it is not so perfect right now. It’s design is simple enough to run on most computers, tablets or smart phone, but it is a great help for those who are not online 24×7.