The following are a few websites I found while browsing the social bookmarking sites Delicious and Diigo.
The tags I used to find all three of these websites were "Elementary" and "Music". It seems to me that a tag is like a keyword you type in to a search engine.
I explored membership in both Diigo and Delicious and found that Delicious was more user friendly for me. I thought that Diigo was difficult to understand and required more time to understand how to use it than Delicious. So I joined Delicious and added the buttons to my browser.
I think that Delicious and Diigo would be most useful in the classroom as a research tool. Teachers can set up a class account and add tags for students to research different topics. For instance in the music classroom some tags might be: worldmusic, instruments, and composers.
In addition, a site like this will help with lesson planning. Suppose a teacher wants to show a website for a particular lesson. He/she could tag the website to a Delicious or Diigo site for quick access during the lesson to show on an activeboard or screen.