Offline Google Mail

Offline Google Mail is an official Google Chrome extension. It can be found in the Chrome Web Store. It is a project still in Beta that allows Gmail to support offline access.

Official Description
Gmail Offline beta is a Gmail app built to support offline access, allowing mail to be read, responded to, searched and archived without network access. After first start-up, Gmail Offline will automatically synchronize messages and queued actions anytime Chrome is running and an Internet connection is available. More than just an offline app, the streamlined user interface, based off Gmail’s popular tablet interface, provides extremely fast response time with a pure, email focused experience.

To start Offline Gmail after installing, open a new tab in Chrome; In the new tab pane you will see a Gmail Offline Icon. Click on the icon, and Offline Gmail will load.