Which social bookmarking site do you use?

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Social book marking sites are getting popular.They aid in saving your bookmarks/ favorite links online. I have been using one such site 'http://del.icio.us' extensively for storing my links.The are lot more and its free.To every link u would need to add a tag related to the links' contents I truly laud the efforts of creating such sites.This is definitely a great way to organize links and contents on the internet.This was demonstrated when I did a search for 'javascript tutorials' on one such site.Believe me! the search results were better than google's in many cases. Which social bookmarking site do you use and what new features has it got ?