Social bookmarking websites are centralized online services which allow users to store and share Internet bookmarks. Such sites typically offer a blend of social and organizational tools, such as annotation, categorization, folksonomy-based tagging, social cataloging, commenting, and interface with other kinds of services like citation management software and social networking sites.
BibSonomy: BibSonomy is a system for sharing bookmarks and lists of literature.
CiteULike: CiteULike is a web service which allows users to save and share citations to academic papers.
Delicious: The site was bought by Avos Systems on April 27, 2011 though was operated by Yahoo! until July 2011.
Digg: Digg is a news aggregator with an editorially driven front page
Diigo: Designed to bookmark web pages and highlight key points for reference. Free.
Google Bookmarks: Bookmarking service by Google
Pearltrees: Collaborative bookmark exploration and curation tool organized and presented like a mind map.
Pinboard: Pinboard can automatically import your links and tweets from a number of outside services. The archive feature saves a local copy of everything you bookmark in case the original page goes offline, and offers full-text search. Currently 11 USD annual fee, with an optional 25 USD fee for archiving.
Pinterest: Pinterest is a web and mobile application company that offers a visual discovery, collection, sharing, and storage tool.
Reddit: Users submit content in the form of either a link or a text ("self") post. Links and content can be voted on.
SiteBar
StumbleUpon: StumbleUpon is a discovery engine that finds and recommends web content to its users.
We Heart It We Heart It is an image-based social network for inspiring images
Pocket: Bookmarking articles and links.
Xmarks: Syncs bookmarks (folders & sub-folders) and profiles across different browsers and platforms. Includes sharing option and possibility to access bookmarks via web-based-interface.
I hope this helps.