You can buy water jugs for a few dollars and they can be used in a number of different exercises ranging from farmers' walks, to lunges, squats and overhead pressing. The water sloshing around in the bottle will make the exercise even more difficult and recruit more muscle fibers. You can also fill the bottles with sand and adjusting the weight is easy. Have you used water jugs for exercise?
I have used small milk jugs as weights. I added some stones to give the jug weight and found that they worked just fine. I have also used soup cans as weights.
I don't usually. I tend to use the water jugs for the water itself. I have, however, used milk jugs filled with water or sand before for exercise, which is the same idea. I can see how people would benefit from water jugs for exercise. You're right about them being pretty cheap, too.
No, and it's the first time I've heard of jugs being used for exercise. That's quite a creative improvisation though.