At Google Developer Day 2007 on Thursday, Google is launching Google Gears, an "open source browser extension that enables web applications to provide offline functionality using JavaScript APIs". Gears works across all platforms: Macs, Windows, and Linux with Firefox or IE, providing the ability to store and serve application resources locally, store data locally in a fully-searchable relational database and run asynchronous Javascript to improve application responsiveness. In short: it's an easy way for developers to create web applications that work offline.
The first app to be "gears enabled" will be Google Reader, which will grab all your feeds when you're online, and continue to work once you're offline. If you read all your feeds while you're disconnected, Reader will sync up once you reconnect, meaning all your online feeds are marked as read. This functionality is going to be extremely useful for web office apps that assume 100% connectivity - I can't wait until GMail starts working offline.
It's another welcome release from Google: see the developer site for the technical info.