Comments about Tiffen Tiffen 62mm 8x (0.9) Neutral Density Glass Filter:

I had to do a fair amount of searching before figuring out how people took those nice, smooth waterfall pictures. The neutral density filters are the answer -- like sunglasses for your camera, so it lets you take longer exposures.

This one is 0.9; each 0.1 is a third-stop (one notch for aperture or exposure), so they're usually sold in 0.3 increments (0.3 = one full-stop).

It works beautifully! It "only" slowed my pictures down to 0.5-1.5sec exposure for bright daylight, but even that was enough to get some nice, smooth pictures of waterfalls. should work really well for streams and other flowing water, too.