Delicious

Del.icio.us (Acquired by Yahoo!) was created in 2003 by Joshua Schachter as a way to organize and share his online content consumption. After opening up the service to allow other users to do the same, the service grew to 100,000 users. Del.icio.us, the leading "social bookmarking" service, allows users to "tag" websites, categorizing them with their own personal descriptions. Since the tags are public, any user can discover new content by searching the aggregated list of links associated with any tag in the system, as well as subscribe to it via RSS. Tagging is quickly becoming a widely used method for navigation and discovery on the web.