Bike trailers rock. If you want to run errands or go biking camping, trailers can be a great assist. Tom, or istomtom on Flickr, sent along a set of pics on a trailer he built himself. I especially like the fenders/wheel wells to protect from side swiping various stationary objects. For the full Flickr set, go here.
A comment from one of his photos states:
In response to comments: MIG welded thinwall EMT. I got the tires for free so I'm not complaining. I haven't decided on a floor yet, maybe steel mesh, fiberglass, fabric, or a slatted wood bed like an old style pickup truck.