I've written about a bunch of screen capture tools over the years. Webpage Screenshot is another good one to add to the list. Webpage Screenshot is a Chrome extension that you can use to capture all or part of webpage. You can draw and type on screenshots you make with Webpage Screenshot. One feature of Webpage Screenshot that is quite helpful is the option to capture the entire content of webpage even if it doesn't appear in your current view of the webapge. See it in action in the short demo video that I made and embedded below.
Applications for Education
Whenever I'm introducing a new web-based service to students or colleagues, I find that having annotated screenshots available to them in either digital or printed form is very helpful. Having annotated screenshots available can help you avoid answering the same question ten times and also allows people who are more comfortable to go ahead on their own. The annotated screenshots also provide a nice reference for students and colleagues when they're working on their own without immediate access to an instructor.