Google recently presented Gears, a way to enable offline web applications.

Google Gears is an open source browser extension that lets developers create web applications that can run offline. Gears provides three key features:

A local server, to cache and serve application resources (HTML, JavaScript, images, etc.) without needing to contact a server

A database, to store and access data from within the browser

A worker thread pool, to make web applications more responsive by performing expensive operations in the background

Google Gears is currently an early-access developers' release. It is not yet intended for use by real users in production applications at this time.

You can check it out at Google Gears.