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Portaportal is a web based bookmarking utility that lets you store links to your favorite websites online. Now your bookmarks are no longer limited to any one machine.
Middlespot is an innovative Web stie for social bookmarking that allows users to collect sites in a visual way.

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Diigo is one of my newest favorite Web tools. Not only can you use Diigo to save your favorite Web bookmarks, yalso ou can use Diigo to put sticky notes on websites, highlight stuff you like, comment, and more.
external image delicious.pngdel.icio.us is a social bookmarking site that has rapidly taken off even more in popularity since being purchased by Yahoo. I use del.icio.us probably the most out of all these top picks, particularly to post interesting sites that I've found around the Web. I also subscribe to quite a few del.icio.us tags, such as popular and reference, and I get all the sites tagged with these keywords in my RSS reader of choice, Bloglines.

delicious tutorial
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Digg is social bookmarking and social networking gone a little crazy due to the fact that anyone can submit a Digg (site), and then anyone can comment on those same Diggs. For me the most interesting feature of Digg has to actually be the comments on the sites and stories, since the Digg community is not shy about letting folks know how they feel about a particular Digg. Primarily focussed on geeky, technology-type items.
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Reddit is a social bookmarking Web site that works much like Furl and del.icio.us: you register a username and password, and then start submitting and sharing your bookmarks. Reddit is similar to del.icio.us in that users are encouraged to vote on the links and stories that they feel are deserving of being in the top dog spot: it’s kind of a popularity contest, so to speak.
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With Clipmarks, you can clip the best parts of web pages. Whether it's a paragraph, sentence, image or video, you can capture just the pieces you want without having to bookmark the entire page.
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The beauty of StumbleUpon, to my mind: you are able to take advantage of a vast network of dedicated Web searchers who are finding utterly brilliant sites and sharing them with you. I do have to warn you, though – StumbleUpon is an incredibly addicting way to search the Web.


Some information referenced from Social Bookmarking Sites -The Top Ten Social Bookmarking Sites on the Web
By Wendy Boswell, About.com Guide