They also feature in some of the smaller ready-made flash systems. The Elinchrom company make a number of two-head kits that feature theses light modifiers. They are inexpensive, easy to carry, and foolproof.
Strobist kits also carry them - you are enjoined by the makers to fire your speed lights into them and reflect the light onto your subject as a wide, soft illumination. By and large they work every time.
The heading image is possibly the laziest product shot I have ever taken, as I did not move from the editorial swivel chair to do it. It shows the rack we have dedicated to Promaster umbrellas. These have a standard diameter shaft ( As opposed to the slightly smaller Elinchrom umbrellas) and are suitable for all sorts of studio monoblocks and speed light brackets. You can get quite large ones - up to 72 inches.
There is quite a variety of construction as well - the basic black outside is standard but you can get the interior with a white or a silver finish. The white has a softer illumination - the silver harder and more specular. There are shoot-through umbrellas that are a very soft modifier and some that amount to a soft box. Admittedly it works backwards as you fire your flash into it away from the subject and depend upon the reflective surface and a diffuser panel inside the umbrella to spread the light.
Best of all with any of them is the fact that you can go out to a job in the field with them collapsed and rolled up and then erect them in a second when you are in position. Soft boxes are never this easy - you either have to assemble them on he site with much bending of arms and puffing and cursing, or take them assembled in the car. They never fit easily in any car...
They are cheap, compared to soft boxes. If you have a need for soft light anywhere I can readily recommend one. In store now.