Sometimes you want a sofa and sometimes you want just a comfy chair, and sometimes you really need an extra bed for guests. The Curt Sofasystem from Ambivalenz is a system of modular furniture that can be all that and more. The basic unit is a "pouf," which can be an ottoman on its own, or a daybed with two side-by-side, or with more poufs on end, can be a couch or an armchair.
The genius in the system is the attachment. Each pouf comes with adjustable belts that can be buckled to another pouf, keeping the shape of whatever furniture you are designing, but easily detached when you change your mind. When they aren't needed, they stay tucked under the fabric cover.
Oh, about the covers. They come in a variety of fabrics and colors, and zip onto the poufs. You can change the colors of the poufs simply by unzipping and changing out the covers. You can even combine different colors!