Google has launched a beta of Google Gears. Google Gears is a plugin for your web browser that allows web applications to store some data on your computer. Without Google Gears this is only possible with the use of (flash) cookies and the like, which can only store small amounts of data. Future versions of browsers, at least Firefox 3.0 will also have support for client-side storage for web applications.
So what is it good for? Off-line web applications. So far the only (major) web application that uses Google Gears is Google Reader. If you have Gears installed you can now download 2000 RSS items locally so you can read it offline. Google Reader with that got an offline mode. When you go back online the changes will be synced back to Google Reader.
Anybody can use Google Gears in their application. APIs are described on the Google Code site. Gears consists of three components:
Database: a full relational database (SQLite) on the client side.
WorkerPool: “Make your web applications more responsive by performing resource-intensive operations asynchronously”
Another interesting development for AJAX web application development.