Lacuna (formerly “Lacuna Stories”) is an open-source, online learning tool designed to create new possibilities for reading and learning collaboratively. By focusing on tools for digital annotation, Lacuna allows instructors, students, and co-learners to discover different ways of reading, interpreting, and discussing course materials. Lacuna enhances the rich, discussion-based learning of a seminar style course, for instance, by extending that conversation to the readings before and after class. The result is often students who are better prepared for class, already have some questions formed (or answered). Instructors can also get a better sense of student interest and can identify “hot spots” that are ripe for discussion or clarification based on student annotations.