This app by Riley Lark, et. al. is nice for gathering data or engaging students. You visit the app at http://activeprompt.herokuapp.com/ and upload an image. Students visit a link (Symbaloo, anyone?) and place a dot on the image according to your prompt (where will these lines intersect? Which best represents your solution? the prompts are limitless!). Then a teacher link shows the class’s dots compiled in one place.
My test image was a tree with the prompt “drag the dot to place an ornament on the tree.” Just needed to see if it worked.