Snoot - A snoot is designed to concentrate light into one small point, usually used to direct light to a particular accessory or as a hair light.
Grids/Honey Combs - Come in varient different intensities and sizes, again similar to a snoot, designed to concentrate light into a particular area, or as a hair light.
Umbrella - A very popular defused, particularly with location photographers, just to the size and ease of transport. Usually chosen for illuminating a large subject, i.e. a full body shot. When the light goes into the umbrella, it spreads out and either shoots through, or bounces out causing a large light source. If you wanted to concentrate the light with an umbrella, simply closing it down is an effective method.
Soft box - Come in varient of sizes, a very popular studio accessory, shooting through a soft box gives a very smooth end result. Usually used for just head/shoulders shots, but as mentioned, some soft boxes are very big, and will easily illuminate a full body.
Beauty Dish - Light hits the middle of the beauty dish, reflects backwards and hits the ‘dish’ and this is pushed out towards the models face. A very popular light modifier, and a very unique look. The light is very centre weighted, but drops off very quickly, so unless the model is stood close to the background, it is usually very dark.
Barn Doors - Very similar to snoots and grids, by opening and closing the doors, you can concentrate or spread out the light coming through.