Offline Gmail was one of the greatest promises of Google Gears, and it's finally available in Gmail Labs—with a few unexpected extras.
In the new offline mode, Gmail will open a cached version of the site—a downloaded in-browser client, basically—and let you browse, write, delete and queue messages for sending (a small indicator icon in the top right corner will warn you that you aren't connected). As soon as you switch to online mode, your actions are synced with Google's servers.
That's not really surprising—it's how all Gears apps work. The cool part is the 'unreliable connection' mode, for situations when your internet access is patchy and inconsistent. In this version, Gmail behaves as it would offline, but constantly scans in the background for a working internet connection. When it finds one, it immediately syncs whatever changes you've made and checks for new mail.
Since all of these add-ons rely on Gears, they will only work in Firefox and IE with the extension installed. You may also need to wait for Offline Mode to appear in your Labs tab in the Gmail settings—the team says that the US and UK rollout, which has already begun, will take a few days. [Gmail Blog—Thanks, Niranjan!]