The line between cloud-based and desktop applications seems to keep blurring. This time, Google has introduced an experimental feature that allows Gmail to work offline—search and all. Best of all, the feature synchronizes changes you made while offline when you get back on the Internet.
Offline Gmail relies on the Gears browser extension, which only works in Firefox and Internet Explorer for Windows, for the time being. Essentially, the tool downloads a "local cache" of your e-mail, which it keeps updated whenever you're online:
Since this is an experimental feature still, you'll find it in the Google Labs tab of Gmail's settings page. Once you enable offline Gmail, you'll be able to go online and offline by clicking an icon in the upper right corner next to your user name.