I decided I needed a bike trailer to haul stuff like groceries, tools, etc. around the neighborhood, and I found a simple plan on Instructables. Mostly I have used it to peddle my cucumbers, lettuce, and pickles to neighbors. I've had a few issues with getting the wheels adjusted right, but it is pretty reliable, and customers like the rustic appearance.
The instructions are a bit hard to follow. Also, I modified several things, based on what materials I had, etc., and made a better hitch than the one they had.