Diigo is a social bookmarking tool, but so much more. In addition to letting you bookmark pages, you can also annotate them. There are two tools you can use for this, highlighting and comments. Highlighting lets you highlight the actual text on a web page, and stores the highlighted words with your bookmark.Think of how useful this can be for online reading assignments in a class (no wonder it’s caught on with some high school AP teachers).
In addition to highlighting text, you can leave comments behind, and even position them using floating sticky notes. You can use these to direct students to specific part of a page, or leave vocabulary or other tips explaining parts of text that may be a little complex for students. Just go to Diigo, sign up and download the Diigo toolbar (or lighter Diigolet bookmarklet)