Google has released a beta version of Google Gears which is an open source browser extension which  enables web applications to provide offline functionality using following JavaScript APIs.

Here’s what you can do with the API’s:

• Store and serve application resources locally
• Store data locally in a fully-searchable relational database
• Run asynchronous Javascript to improve application responsiveness

You can take a look at the sample applications at http://code.google.com/apis/gears/sample.html once you install the browser plugin.

The news out there is that Google’s planning to offer offline support for their current applications like Google Reader, Docs & Spreadsheets.