Have you always wanted a document camera (example: ELMO) in your classroom? If you have a Tablet (LearnPad, Android, Windows, iPad) you already have a document camera!
Instead of shelling out more money on a document camera; use the Tablet you already own. The picture below shows a tablet on a stand. You can also setup a stack of books or something similar and have the tablet's camera hang over the edge. Be sure to keep students away from the area when you are using the document camera so there aren't any accidental bumps. Using software like AirServer, you can project the tablet's image to the SmartBoard.