feel and quick functionality of desktop-based photo-management applications, such as Google's Picasa and F-Spot. Users can select and apply changes to multiple photos at a time, as an alternative to the standard Flickr interface for editing. Access control Flickr provides both private and public image storage. A user uploading an image can set privacy controls that determine who can view the image. A photo can be flagged as either public or private. Private images are visible by default only to the uploader, but they can also be marked as viewable by friends and/or family. Privacy settings also can be decided by adding photographs from a user's photostream to a "group pool". If a group is private all the members of that group can see the photo. If a group is public the photo becomes public as well. Flickr also provides a "contact list" which can be used to control image access for a specific set of users in a way similar to that of LiveJournal. In November 2006, Flickr created a "guest pass" system that allows private photos to be shared with non-Flickr members. This setting allows sets or all photos under a certain privacy category (friends or family) to be shared. Many members allow their photos to be viewed by anyone, forming a large collaborative database of categorized photos. By default, other members can leave comments about any image they have permission to view and, in many cases, can add to the list of tags associated with an image. Interaction and compatibility The core functionality of the site relies on standard HTML and HTTP features, allowing for wide compatibility among platforms and browsers; Flickr's functionality includes RSS and Atom feeds and an API that enables independent programmers to expand its services. This includes a large number of third-party Greasemonkey scripts that enhance and extend the functionality of Flickr. In 2006, Flickr was the second most extended site on userscripts.org. Organizr and most of Flickr's other text-editing and tagging interfaces use Ajax, with which most modern browsers are compliant. Images can be posted to the user's photostream via email attachments, which enables direct uploads from many cameraphones and applications. Flickr uses the Geo microformat on over 3 million geotagged images. According to the company, Flickr is hosted on 62 databases across 124 servers, with about 800,000 user accounts per pair of servers. Based on information compiled by highscalability.com, the MySQL