Waveboard
Since Waveboard looks basically identical to Mellotron, I won't put a picture of it here. It essentially does exactly what Mellotron does, but rather than looping every instrument's sounds, it plays them once, when a key is struck. This is useful for instruments that don't sustain infinitely, like pianos, guitars, and drumsets. Likewise, Mellotron is better suited for organs, synthesizers, flutes, and string instruments. You are also able to define your own instruments in Waveboard just as you could with Mellotron.