Google has made offline versions of Gmail, Calendar and Docs

You can use Gmail offline today, and offline versions of Google Calendar and Google Docs will be available during the week starting today.

Gmail Offline is an application in the Chrome web store, designed for situations where you need to read, reply, organize or archive mail without an internet connection. This HTML5 application is based on a Gmail for tablets web application that was designed to work with and without Internet access.

After installing the Gmail Offline application from the Chrome Web Store, you can continue to use Gmail when the connection is lost by clicking the Gmail Offline icon on the tab with a new page in Chrome.
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In Google Calendar and Google Docs, you can go from online to offline mode and vice versa. To use them offline, simply click the gear icon in the upper right corner of the web application and select the offline access option.

While offline in the Google Calendar, you can view calendar events and respond to invitations. In Google Docs, when there is no connection, you can view documents and spreadsheets. Offline editing is not ready yet, but Google is working on it.

Google promises to make support for other browsers as soon as they start supporting additional features (for example, background pages).

via Official Gmail Blog