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.
If find a page interesting and want to save for future reference or share it with other people, simply click on one of these links to add to your list.
All of social sites listed are free to use, but do require you to register. Once you have registered you can begin bookmarking.
You can find out more about social bookmarking on Wikipedia.