BibSonomy
"A system for sharing bookmarks and lists of literature" (Description from website). Bookmark websites, and post reference information about publications (articles, books, etc.). You can attach pdf's of articles to references. You can also search the BibSonomy database for publications, and see the number of people who have posted that paricular publication in their own libraries. Not compatible with word processing software to add reference information to a document.

Delicious
A "free, easy tool to save, organize, and discover interesting links on the web" (Description from website). Shows the number of other users who have bookmarked a particular webiste.
Diigo
Diigo stands for Digest of Internet Information, Groups and Other stuff. It is a website for bookmarking and sharing webpages.
EndNote Online
Save references, import references from databases, automatically insert footnotes and create bibliographies for research papers, and more. Share your references with fellow researchers who also have EndNote Online accounts.