Steve Bodiley had something he needed to carry with him whilst atop his velocipede, presumably another person or pet, or perhaps some groceries or supplies. And rather than taking the behind-the-bike cart option, he took a hint from the motorcycle crowd and created a side car.
The design features a welded frame and a custom wooden body, but I'm thinking this one's repeatable even for those of us whom don't have access to welding gear/knowledge. Some galavanized pipes would do for the base, methinks, and the plywood skin looks easy enough to recreate.
See how everything was done at Steve's Workshop.