Diigo is a free online bookmarking tool that might not be as slick looking as newer services, but it is still a great tool for students and teachers. Diigo lets you bookmark websites on any device. You can tag your bookmarks and add notes to them to remind you of why you saved a link. Beyond the bookmarking tools, there are some other neat features available to students and teachers who create Diigo accounts.
Groups is my favorite feature in Diigo. You can create public and private groups within which you share bookmarks with collaborators. I like to make Diigo Groups and have students share their bookmarks with each other. Doing this can help students find more resources than if they worked alone without sharing resources. In the video embedded below I demonstrate how to create a Diigo Group.
Diigo's Outliner tool provides students with a good way to organize their bookmarks into a package that supports their research projects. For example, if a student was working on a research paper about the Electoral College he could use the Outliner to organize all of his resources into a format that supports his writing process. Beth Holland provides a detailed overview of the Diigo Outliner in this post.
Last, but not least, students can use Diigo to add annotations to their bookmarks. In the video embedded below I provide directions for doing that in Firefox. The same can be done in Google Chrome.