Annotate the web with highlights and comments. Add hyperlinks to your answers, and reply to other people's comments. You can create both private and public annotations for yourself, or create a group where your students can create their own annotations.
Free icons available under a Creative Commons Attribution-NoDerivs 3.0 Unported, no registration required. You can edit your icons, changing color, overlaying other icons, adding text, and more (even in the free version).