Gmail Labs added a new features “offline Gmail” that once enabled will load Gmail in your browser even when you don't have an Internet connection. You can read messages, star, label and archive them, compose new mail and more. Messages ready to be sent will wait in your Outbox until you're online again. It's built on the Gears platform, which has already been used to offline-enable Google Docs, Google Reader, and a number of other third-party web applications.

Here're the steps:

1. Sign in to Gmail and click 'Settings'.
2. Click the 'Labs' tab and select 'Enable' next to 'Offline Gmail'.
3. Click 'Save Changes.'
4. In the upper righthand corner of your account, next to your username, there will be a new 'Offline' link. Click this link to start the offline synchronization process.

More info: Gmail Blog