Flute Air Stream Visualizers
One of the most common problems every beginning flutist struggles with is how best to guide their air stream into the instrument.  A strange looking yet quite effective tool for teaching this skill is known as the Blocki Pneumo Pro.  Essentially it is a small arm that attaches to the flute and has on it several tiny little pinwheels.  A flutist practices guiding their air stream toward each of these pinwheels, making them spin.  With a little guidance the student begins to improve their tone quality and range.  More advanced students use the device to practice double and triple tonguing as well as tuning and intonation issues.  The device sells for around $30 and is a nice visual way to show a student what they are doing right or wrong during a lesson.