I already had a strobist set: 2 light stands, umbrella mount and a set of radio-slaves "Cactus". After moving I ended up with an additional room, which I decided to convert to a studio. But I had no extra money for that.
First thing that I needed was a background. Actually two, the black and white. Following an advice from a forum I went to a hardware store and bought some felted cloth, with a surplus. That was already a start, but umbrellas reflect to all asides and I wanted something that would look like a strip.
As a base I took a shipping box that came with a fridge (2 meters high). I cut the box diagonally and got 2 big leafs. The third side I made out of the surplus of a white cloth. Stapled it. Glued some rumbled foil to the cardboard on the inside for better light dispersion. On the opposite side from the cloth I made a hole for a flash (1.5m heigh from the floor) tight enough for a flash head to just go in and stay fixed.
The strip stands on the floor by itself and is quite stable. It doesn't require a lightstand nor a flash mount. Besides you can easily fold it and put it in (under or behind) a closet, just don't forget to take out the flash. the working surface width can be adjusted from 10 to 50 cms. And you get whole 2 strips from a single box. The outer sides of the cardboard are covered with the surplus of the cloth, also with staples. First strip has white left side, and black right side. And the second one in the reverse (black left, white right). If you place them black side to the camera when shooting on a black background (or white for a white background) then they are invisible. And also you can use them without a flash to block or disperse light.