Google Gears
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.