MoocNote is a tool (web based or Chrome extension) for adding time-stamping annotations to online video. Educators could use MoocNote to add comments, questions, or links to videos students are assigned to study or could be used by a student to mark spots within a video (with comments) that they might want to quickly identify for reference or study. I am unclear on the MoocNote plans offered to users. There are both a free and premium plan with different capabilities, but the premium version is listed as being in beta (I first used MoocNote several years ago so this seems strange). The following videos describe the setup and application of this service.
MoocNote is an example of what I describe as a layering service. What I mean by this is that the service allows the layering of elements to existing online content without actually modifying the original content. This is important in preserving the expectations of the content creator in that his/her material is still served as expected to users.