Xtracycle makes a brilliant load carrying system for regular bicycles. A travois is a Native American conveyance in which 2 poles are yoked to a horse or dog, with cargo and passengers sitting upon a platform between them: a dragged A-frame. Xtravois is inspired by both. It's been the family car since late 2003, and I think the first Xtracycle-compatible longtail frame anywhere.
There's a video clip (6.2MB) available commemorating a bright spot of the highly technical design process. The De Cecco pasta girl nearly made it onto the headtube.
I designed Xtravois as a done-right successor to this hack (seen in the first photo), which blew up after about 5K miles. I went on to develop the electric assist component into a product called Stokemonkey for nearly any regular Xtracycle.
Many people ask if I am selling frames like this, or if I have build-ready plans available. No. See the Surly Big Dummy, though.