Social bookmarking is a way of bookmarking your favorite web sites and sharing them with other people. You can even make notes and annotate your bookmarks. It's a great tool for education, allowing teachers to share web sites with their students very easily. Teachers can add notes, or even questions, to the bookmark to make it an assignment or explain the content better.
There are three main tools I've used for sharing bookmarks: delicious, Diigo, and Google Bookmarks. All three have various features that make them useful. However, word has just spread this week that delicious is closing up shop. Here's some more info:
Delicious, the social bookmarking service that is widely used (including in schools) is closing. This news has quickly spread to throughout the internet and while there is a push for Yahoo (who owns Delicious) to keep it open, many people are offering up solutions for people who will be losing their bookmarks.
Diigo, another social bookmarking service, has the ability for you to import your Delicious bookmarks into Diigo. See article below on how to do this.
Diigo is a really powerful tool, with groups you can follow, browser extensions to make it easier to share sites you bookmark, and even annotation tools. With it, you can easily annotate and share bookmarks with your colleagues or students. Students can set up an account and share with others also. You can even search in groups and find other educators and see what bookmarks they are posting.
Google Bookmarks is a free service from Google that allows you to keep your bookmarks in their system so that you can access them from anywhere. You can sort and tag them by subject and even share them with others. I use Google bookmarks the most, but share bookmarks with Diigo.
Here are some great articles on Delicious, Diigo, and Social Bookmarking:
Cool Cat Teacher - great article on social bookmarking
Delicious is Closing...how to import your bookmarks into Diigo in 5 minutes.