Extremely cool:

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

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

From the FAQ:

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

The database is SQLite, everything is access though JavaScript. BSD-licensed.