CETMA 5-Rail Cargo Rack
These racks, hand-made by a couple of welders in the US, are favourites of bike messengers worldwide. They have a large cargo area (measuring 28cm square) that's perfect for boxes or bags.
Attaches to the handlebars and the axle. You can adjust the height/distance of the attachment points for both.
Fits bikes with 12" or less between the handlebars and the front tyre.
Maximum weight: as much as you can fit on it.