Ask3 “is a tool for quickly sharing text and audio between teachers and students. It allows a user to turn an iPad into a whiteboard, record voice and text to either ask a question or get feedback.”
“Conojo is an innovative drawing tool that makes it easier to share ideas, images, notes, and any other visual manifestation you can think of.” It is not free: It costs $2.99. There are lots of these apps you can download free.
An interactive white board and screencasting tool used by over a million teachers and students.