I'm one of those photographers that likes to take control, especially of my light. I use grids, snoots, barndoors, and every other contraption you can think of to maintain the maximum amount of control over my lighting. One of the most important light modifiers for my work isn't a soft box or a beauty dish, it's actually a piece of fabric on a metal frame called, a flag.
This video, by The Lights Film School, does an incredible job explaining the incredible value flags have when lighting a scene. They go over how the light changes depending on the position of the flag, not only in relation to the light source, but to the subject and the background scene. The tutorial is given from a video perspective, but it all applies to stills as well.
Do you use flags in your work, if so tell us how you like to utilize these wonderfully simple pieces of gear and better yet, post some examples!