1930: Overhead Projector.

It got its start pre-WWII, when the Army used it for training soliders. Then, the police got interested, because it made identifying suspects easier. You could project images across a screen. By the late 1950s and early 1960s, overhead projectors made it into schools, and the interactive classroom was born.

Teachers liked them because they could use pre-printed transparencies for lessons, and enlarge them easily. No more need to write everything in extra-large on the blackboard. In most classrooms computers have taken their place.