Clickers
aka classroom response systems (CRS), aka audience response systems (ARS)

These are little remote control-like devices that each student purchases (or is provided). The teacher asks a question, and the students enter their answer, and the teacher can display a graph of the answers. This is a type of formative assessment. There is a good bit of research now showing that clickers (as well as other kinds of formative assessment) are very effective if used correctly.