Yesterday during the Google Developer Conference, Google announced Google Gears.

Google Gears (BETA) is an open source browser extension that enables web applications to provide offline functionality using following JavaScript APIs

Have a look at the examples that run  with Google Gears  :

• Simple Database Demo - Store and retrieve values in the database
• Simple ResourceStore Demo - Capture a URL and display it while offline
• Simple ManagedResourceStore Demo - Capture a versioned set of URLs atomically and update them together
• Simple WorkerPool Demo - Perform a long-running computation without blocking the browser UI