Without a tripod, the pictures below would not have been possible. This is because we typically use shutter speeds of 2 seconds or more, which is not possible to hand-hold without getting camera shake.
Polarizing Filter or ND Filter
Circular polarizing filters are designed to reduce reflections and increase saturation. As a side effect, they also cut down about 1.5 stops of light coming through the lens.
ND filters are neutral density filters, available in different strengths. They act like sunglasses for your lens by cutting down the light coming into the lens.
Timing and Location
Vary your shutter speeds for different moods. There are many other variables to fine-tune, like composition, color balance and foreground interest. So keep on experimenting to get the perfect waterfall shot.
You can also apply this technique on other water surfaces, like lakes and dams. Ripples on a lake can be smoothened out by using a long exposure. The result is a mirror-like calmness. Reflections on this surface are sometimes rendered as wavy surreal reflections. This was exactly the technique used for the 2 images below.