I came across this site while looking for a bike trailer that doesn't have the draw bar off to one side like most seem to do. These ones all attach to the seat post.
Here is a good site with a wide selection of trailers. Most are utility (the largest of which, has an 800lb capacity), though there are those that will let you tow your kayaks, your groceries or even your dog. The trailers are made in Canada, on Vancouver Island.
Website:
http://www.tonystrailers.com/