For those who don't have one or haven't used one before, these are handy dandy little light modifiers. A grid is simply a snoot that has honeycomb cells that will give you a small circle of light on your subject, your background, or for a rim light on your subject.
It is similar to a snoot in looks and function but is a bit more restrictive and gives you more control over your light.
A grid is perfect for helping you to create dramatic lighting effects.
Here are a bunch of shots taken with grids, they will give you an idea of the possibilities.
So here are six different (easy to make) DIY grid projects, three made with straws, and three made with coroplast (a plastic corrugated cardboard):
The straw projects are from Lighting Mods (part 1 & part 2), LightChasers Photography, and Stephen Zeller.
The coroplast grids can be found at Instrucatables, Pamela Helme, and DIY Photography.
Now go make a grid (or two)!