Perhaps on a related topic, if you do not have any ND filters and do not want to spend much, there are ways around it.
1. Circular Polarizer. They usually drop light by about 2 stops slowing your shutter speed. Also handy for reflections, really vibrant colours, blue skies, etc. This would be most used filter.
2. Welder's Glass. Produce horrid colour casts so you must shoot raw and fix on the computer. Quality will often lack. Need to improvise holding in front of lens. (elastics)
3. Inexpensive ND filters. Can go circular or square. Square need to have holders or improvise as above. Circular of course are difference sizes, but you can get the largest you would need and then get cheap stepping rings to go down in size for other lenses.
The April Digital Photo has a test on inexpensive ND filters. Their top choice is from here http://www.srb-photographic.co.uk/
I was recently recommended these by a few in another forum.
One more thing..variable ND filters. These are circular and rotate to adjust how much light they block. How they work is that they are two polarizers together. They are ok for small amounts of light but they are never good to block lots, they tend to get an X in the middle of different light. I bought a cheap on to play with.