Those lucky enough to live by the sea can take advantage of nature's bounty when the seaweed's thrown ashore at the end of the year. It isn't a soil improver in the same way as bulky organics, but as a growth stimulator it seems to have no equal. I mulch with it in late autumn and early winter and let the worms do the rest - worms which subsequently show up dressed in incredibly vivid colours. if it does the worms this much good just imagine what it's doing for the plants!
Plants grown on areas which have been "seaweeded" are not only way more vigorous and healthy, but show much improved pest resistance. I read somewhere that it's slightly alkaline and the effect on my clay soil seems to bear this out, with a more crumbly texture similar to that achieved by liming.
The stuff doesn't add humus to the soil, so I still add bulky organics at mulching time, generally my own compost, or occasional applications of horse poop from a local source.