Neutral Density Filters

Not to be confused with graduated neutral density filters, Neutral Density (ND) filters simply block light without changing the colors, hence the neutral part. You can do all sorts of creative things with these filters. The idea behind blocking light is to use much longer shutter speeds. This lets you record the passage of time in a single image. It works wonders for turning ocean waves into ethereal ground fog, smoothing waterfalls on bright days, showing movement around a still object, and more.
By using these filters you're also giving yourself more creative control with depth of field. Normally you'd need to stop the lens all the way down to get long shutter speeds. With neutral density filters like Singh-Ray's Variable ND, you have a wide range of densities available, meaning you may not need to stop the lens down more than you want to, thus giving you better control of depth of field and your backgrounds.
Below are just a couple of examples. The gull image was shot at 1 second at f/20 and the fence image was 4 seconds at f/22. But dialing in your shutter speed all depends on the situation, so don't use these settings as a template. Fast moving surf or strong winds require shorter shutter speeds than slow surf or light winds would. These were also shot at my lowest ISO setting so that I could achieve longer shutter speeds.

Click on the image below to see the other image. I'll try to post more of these along with some before and after examples.