I use a Bykaboose Gecko. It works great for hauling cargo around. The wheels can be removed and it folds flat in about a minute. Minimal assembly required. It mounts on a "hitch" that attaches through the rear wheel skewer.
It seems to go for about $200, but I won mine from a raffle.
http://www.bykaboose.com/
http://www.bikemania.biz/ByKaboose_G...oose_gecko.htm