The Kirk Hugger is available in two sizes, each of which can come empty or filled with poly pellets. You are sure to find these support bags an indispensible addition to your equipment. The Kirk Hugger is made of durable black Cordura nylon with a zip closure. In a pinch you can make your own bean bag by filling a ziplock bag with coffee beans or sand. Hiking staffs sometimes have tripod mounts on top, serving double-duty.
Wind River makes some nice wooden ones.
Manufacturers Gitzo Benbo prides themselves on making tripods that can be set up and used on any terrain, in any conditions and in an unlimited range of configurations.