Social bookmarks, what are these?

These sites allow you to store, tag and share links across the internet. You can share these links both with friends and people with similar interests. You can also access your links from any computer you happen to be using.

All of these sites are free to use but do require you to register. Once you have registered you can begin bookmarking.
Each of the sites works slightly differently so use the links below to find out which service best suits your needs.
You can find out more about social bookmarking on Wikipedia.
Wikipedia article on social bookmarking

For more details about each of these services:

Del.icio.us 
To register, go to: http://del.icio.us/register
For more information: http://del.icio.us/about

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Digg 
To register, go to: http://digg.com/register
For more information: http://digg.com/about

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reddit 
To register, go to: http://reddit.com/login
For more information: http://reddit.com/help/faq

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Facebook 
To register, go to: https://register.facebook.com/r.php
For more information: http://www.facebook.com/about.php

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StumbleUpon 
To register, go to: http://www.stumbleupon.com/sign_up.php

For more information: http://www.stumbleupon.com/about.html