Hypothes.is. An excellent tool for students to do shared annotations of PDFs or webpages in classes. Can be public or in class groups that are private.
A recent panel at the Digital Media and Learning Conference on innovative uses of hypothes.is in teaching.
Jane VG’s Diigo bookmarks about Hypothes.is
The Marginal Syllabus project: Flash annotating of selected readings about education and equity
Fostering Democratic Dialogue with Digital Annotation Essay and video of webinar on using hypothes.is
Forming Groups within Hypothesis:
Perusal Annotation tool highly recommended by Becca.
Vialogues Asynchronous annotations of original or web videos. Can be public or a class group.