Silks or even semi-opaque fabrics and other materials can be placed in front of lights or hung just off-frame for adding texture or reducing light. Silks must be mounted to a frame, stand or other device, but they're not limited by the shape of your light modifier, so they're very versatile for placement. They can also be used as background material with a large enough panel.
Most modern flash designs include a swiveling head and small bounce card for diffusing light output by bouncing it off of a wall or ceiling, which spreads light rays more evenly through a scene. For finer control, you can use much larger reflectors and bounces in a large variety of different materials, sizes and textures.
F.J. Westcott Rapid Box Strip 10' x 24'
These different constructions will influence the reflected light. A silver bounce, for example, is highly reflective, and the light will be very bright and specular like a mirror. Gold, on the other hand, introduces warmth to the reflected light, making it an ideal tool for adding an afternoon glow to portraiture. White bounces are available (you can also use bright white posterboard), but they're not as specular as silver, so the light produced is softer and more diffused for a natural daylight feel. Reflectors and bounces are often malleable for more control over the shape of reflected light and are typically designed as quick-twist round panels that can be handheld or mounted to a stand.
Flashpoint Q Series Snoot
Grids and snoots control rays of light by channeling them. Grids have a namesake "grid" that's placed directly in front of the light, which is channeled through these grids at a straight angle. This eliminates side spill for stronger contrast and a more direct throw. Be wary of cheaper models, as thicker grids will show a patterned grid of shadows in the light.
Snoots are highly directional, even more so than your flash. They provide so much contrast that they result in a spotlight effect with extreme natural vignetting around the light. Snoots are often too powerful, so there are many models that will allow you to shape the front cone for more or less light.
Matthews Flags & Cutters
Though they have less application with the small form factors of flashes, there are also tools to shape light by blocking it. In studios, C-stands and gobo heads are used with extension arms to add light blockers like flags and cutters, which are usually constructed from black fabric or metal to absorb light spill and add shadow, useful for thinning faces and bodies in portraiture. They're also used to prevent directional flares from entering the camera lens from your lights. This gives you a way to control subtle spills of light at the same time that it gives you the ability to keep one portion or subject illuminated differently from another in your shot. Scrims are similar to silks, only they reduce or block light like a flag as they're made from much thicker materials. Like silks, the advantage is versatile placement.
KitExpoImaging Rogue Master Lighting Kit
Cucosloris tools, also known as "cookies," are flags with patterns cut into them in order to facilitate fake shadows or silhouettes, like tree branches or other textures. Gobos are similarly patterned cutouts that are placed within the light housing or directly affixed to the front of a light.
Finally, gels give you a way to produce in-camera color effects while also providing a solution for working under off-white or mixed-lighting situations. The most popular gels for camera flashes are CTO (orange) for matching flash output to tungsten and halogen lights, and Plus Green for working under fluorescents. CTB (blue) will correct your flash output for areas where you have a heavy blue shift like shade. Gels are available in varying strengths as well as kits.