Led by such sites as del.icio.us, social bookmarking is common practice now. Users add their bookmarks to del.icio.us, attach comment to them, and add few tags – key words to describe the content of the bookmarked web page.
Then other people can then search by tag, or view “clouds” which show the list of tags in fonts of different sizes, bigger fonts used for the tag-words used for lot of bookmarks, smaller for tags used for smaller number of bookmarks.
The service called Diigo, goes a step further, it makes it possible for users to highlight parts of the texts, add notes and comments to the highlights (or the whole page) and then share those highlights/comments with other people.
This opens very interesting possibility of group analysis of books and papers on the web which are in html form.
The service is still in Beta, and accepts new users just through invitations. From what I’ve seen it is still buggy, though it is intensively developed.