Symbaloo is a bookmarking service that creates visual bookmarks, lets you organize them, and you can also share them. You can access your bookmarks from any computer.
A free account is available. For more features, you would have to sign up for one of the fee accounts.
As seen above in the Symbaloo Edu page, you can organize your bookmarks in different categories. The Symbaloo Edu page has lots of great links for teachers to use, organized by subject and topic.
SymbalooEdu can be used by teachers to create their own bookmark webmix for classes and for themselves. Students can use it to create bookmark webmixes for their classes.
There is also an EduTools page with links to lots of great resources, an educational news page, and a widgets page with calculator, stocks, tasklist, notepad, and more available to add into your webmix.
It is very useful for education and free.