Google Announces "Gears" Offline Support For Web Applications
What is Google Gears
Store and serve application resources locally
Store data locally in a fully-searchable relational database
Gears works on Macs, Windows, Linux on IE, Firefox. Opera support isn’t finished yet and Safari support will be coming “later.”
They have also released a new version of Google Reader Today which has offline support enabled. You’ll need to install the Gears Extension for your browser to see it in action.
If you are a developer head over to the developers page to see some demos and documentation.