Bob Singh's Variable ND Filter (Geeky)
This is a little geeky; I apologize to the non-geeks.
There are times when I want to shoot slow shutter speeds on a bright day with an open aperture. I've stacked a bunch of Polarizer or ND (Neutral Density) filters, but it was inconvenient and gave a major vignette. Now, Bob Singh at Singh-Ray has a Variable ND filter that lets me dial in up to 8 stops of ND and still preserves color fidelity.
In this picture, I was able to use ISO 3200 at f4.5 to record the bright sunny outdoor and still get a slow 1/20 second shutter speed to blur the car. Good stuff.
The variable ND is also handy for shooting motion footage.