Mellotron

The Mellotron is a polyphonic tape replay keyboard that generates its sound via audio tape. Each key on the Mellotron had recordings of real instruments on a piece of magnetic tape under each note of the 3-octave keyboard and each key had its own pinch roller and playhead. When a key was pressed, the pinch roller enaged with a master capstan wheel and dragged the key's tape over a playhead. Here is a picture from Wikipedia demonstrating this application.

 

Its popularity increased following its use by The Beatles, The Moody Blues and King Crimson, as well as being a notable instrument in progressive rock generally. It was one of the most unique electronic instruments ever made and a trademark sound of '70s prog-rock bands. The following video was part of a Japanese documentary series 'Song to Soul' where Robert Webb talks about King Crimson's making of 'In the Court of the Crimson King'.