Apple launched the Safari Extensions Gallery for it's Safari web browser. In June, Apple opened up the extension functionality to developers prior to this public launch--giving developers some time to create extensions by the time of the official launch.
Extensions are built using HTML5, JavaScript, and CSS3--the technologies that Apple has been championing for the web development.
An important security note: All the extensions are sandboxed, which means that they are unable to access a user's private information or transmit communications with other outside systems.