When I'm not flying around in planes and helicopters doing aerial work, I enjoy shooting seascapes and long exposures. One of the most important pieces of equipment for this type of work is a decent set of Neutral Density (ND) filters. A ND filter is a filter that "reduces or modifies the intensity of all wavelengths of light equally, giving no changes in hue or color rendition" (thanks Wikipedia!). Some of the reasons that you might want to use a ND filter include blurring water, getting a shallow depth of field in bright light, extending long exposures, and so on. They usually come in three forms, solid, split grad, and my favorite, the variable. Solid ND filters are just that, a constant shade of grey across the filter. The darker the grey, the more reduction in light that is allowed to enter the camera. ND filters start at a 1/3 stop reduction, and go up to the massive 10 stop (like the Lee Big Stopper) and more! A 10 stop ND filter is nearly impossible to see through. When using my 10 stop, I will usually set my camera to manual focus, then focus on my subject, and finally carefully screw my 10 stop ND filter onto the front element of my lens. When standing waist deep in cold Maine water shooting a waterfall, I use both hands when screwing the filter on and off! Using my 10 stop has allowed me to shoot 30 second, and sometimes longer, exposures in the middle of the day.
These are usually round glass filters, sized to screw into the front element of your lens, although they also come in rectangular, one size fits most, resin and glass filters. Some of the larger manufacturers are Lee, B+H, Singh Ray, and Cokin. A newer company on the market, Hitech, puts out an inexpensive series of resin filters.
A variation on the round, glass, screw in ND filter is the variable ND filter. This filter is actually 2 pieces of glass, and the more they are rotated, the darker the effect. With more and more photos shooting DSLR video, this variable ND filter can be used to control shutter speed when shooting video, especially on bright days. And because you can adjust the effect just by turning the front piece of glass, you can actually achieve the desired effect while watching your display. Pretty convenient!
Split Grad ND filters have a shade of gray on top and are clear on the bottom. They come as a "hard" or soft" filter. A hard split grad has a sharp transition from ND to clear, and would be used in an instance where there is a sharp horizon line, such as the ocean in a seascape. When that sharp horizon line isn't available, a soft split grad graduates more slowly from the ND effect to clear. You might use one of these for a forest scene with a bright sky that needs some taming.
One last thing to consider is "stacking" your filters. I have often stacked a polarizing filter on top of a ND filter, particularly when shooting waterfalls. The addition of the polarizer allows for a little color saturation, removal of any reflections, and at a loss of 2 more stops of light, for even longer exposures. This is a fun way to get really creative, but keep in mind, that like with most things related to photography, you get what you pay for. Less expensive ND filters may not reduce all of the wavelengths of light equally, resulting in some color shift in your images. My (reasonably) inexpensive 10 stop filter gives me a warm color cast in exposures that last two minutes or more. I can usually reduce, or eliminate this in post, but when you consider that a top of the line 10 stop ND filter cost well over $300, a few extra minutes in post is OK with me. Happy shooting!