Penolo is a cool site that lets you create a sketch or annotate an image, and then share the sketch via Twitter. The images that you want to sketch over can be uploaded or you can enter the image URL. Images can be JPEG, GIF or PNG and can be no larger than 500kb.
This could be a good way for educators or students to share an idea, help, and more via Twitter. I can see using it to use a sketch instead of words to explain or describe something.