Softboxes are often considered a must-have piece of equipment when doing any sort of portrait or studio photography. While they are not the most expensive photography accessory, they can be built far cheaper than you would pay for an off the shelf model.
[Don] needed a softbox for his studio, and he ended up constructing a fairly nice one out of a styrofoam cooler. He mounted an outdoor light receptacle inside the cooler after laying down a reflective backing, bolting everything to a piece of plywood situated on the back of the cooler. He stretched some white cloth over the front to diffuse the light, and then mounted it on a light stand. You can see a video of the construction process below, as well as additional softbox-lit images on his site.
[Aud1073cH] had a similar need for a softbox, but went about his construction a bit differently. He grabbed a lampshade and a white dress shirt at a thrift store, stretching the shirt over the bottom opening before securing it with Velcro. He mounted the lampshade on a light stand, inserting his camera’s speed light through the smaller lampshade opening. As you can see in his photostream, the softbox does a great job at softening the shadows in his pictures.