This effect is partifcularly cool on old electric organs. But you can get simmilar cool effects on melodicas, and conventional synthesizers. Toggle the volume knob up and down. With the organ there are multiple volume knobs for all the synthesized instruments. Many instruments can be toggled at once, and when yo do this you get a natural tremolo ( and sometimes a "dee-dee-dee-dee!").
Another cool thing for keyboarbers, is doing an instrument or "voice" change in the middle of the melody. It's unexplected and can put a totally different flavor on the sound.
If you're looking to record your stuff check out the free music software online called audacity. It will help you in your magical mystery tour.