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.
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.
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.