Social bookmarking sites are free and 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 the sites require you to register before you can begin bookmarking. You can find out more about social bookmarking on Wikipedia

Here are some of the more popular bookmarking services you may want to take a look at.
For example, Facebook has several large groups devoted to horses.

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

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

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

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

StumbleUpon
To register, go to: http://www.stumbleupon.com/sign_up.php
For more information: http://www.stumbleupon.com/about.html