Marqueed that allows users to create and discuss collections of images. This morning I gave the service another look and discovered that you can also use it to share an comment on PDF files. Using Marqueed you can highlight and draw on images as well as PDFs.
Applications for Education
Marqueed could be used to have your students read and comment on a primary source document that you have in PDF form. You could also use Marqueed to upload an image of a diagram and ask your students to label it.