Food colouring dropped into water, shot in slow motion. It is quite long at 5.49, but since it was shot at 60fps (and replayed at 30fps) you can set speed to 200% on your timeline and it should still look ok. Or cut out just the bit you want, of course. About half way through, a second colour is added. The hue is cycled throughout the video.
Shot through the (very dirty) window of the bus home from work, using the casio fc100 camera, at 30 fps and then switching to 310 fps. Used in the Road movie collab: http://www.hitrecord.org/records/51633