I utilize several social bookmarking sites.
Diigo is a social bookmarking site that provides teachers with special provisions to create classroom accounts for free. My personal Diigo account is http://www.diigo.com/user/njtechteacher which primarily contains educational bookmarks.
I created a second account as a teacher. My students began using Diigo in the fifth grade. It enabled them to bookmark websites online so that they could easily do research at home or at school and avoid repeating the web searches. The school Diigo account is https://www.diigo.com/user/sms-cranford.
A group is maintained for technology integration specialists with many contributors: https://groups.diigo.com/group/rcan-shared
Delicious is another social bookmarking website. This is the first social bookmarking website I used. I still maintain the bookmarks through Diigo. https://delicious.com/njtechteacher.
Symbaloo is a tile-based online bookmarking website. I chose this site because it is image-based. Children in the preschool level can readily find the proper bookmark. I am using the free version of the site. I had to embed the Symbaloo pages in a wiki.
This set of tiles has changed over the years as I moved from the computer lab. It is still maintained by the new computer teacher: http://saintmichaelcomputer.wikispaces.com/symbalook.
This page contains tiles I originally used in work with technology integration specialists: http://annoro.wikispaces.com/01+Bookmarks