M.O.D.
More on Diigo … I’ve written about this bookmark tool before. Even though I also use Delicious, I think Diigo offers richer potential for educators, especially for those who teach online (e.g., groups, annotations, avatars). Recently, they’ve enhanced their group sharing feature so you can send links shared in a group to your own blog. So in a learning situation, if one student shares a link with the group that another one finds interesting enough to blog on, this tool allows them to do so seamlessly. In this kind of dynamic, students could not only share information, but build on it via their blog posts, which then other students in the course could read and/or comment on.
Addendum: For OpenID fans, they’ve also added that as a login option.