Back in the days in the beginning of the tall cast iron radiators, they were set in the loop so that the hot water would rise and by the time it went through the radiators in its zone, and by the time it went through the radiators and cooled it was on the downward leg of the run back to the boiler. This would work best in a two story home where it would go through a few on the first floor, a few more on the second floor, so it would be cool enough to make its way back down to the boiler in the basement, repeating the cycle. It is called a gravity system. there is a small article near the bottom of the page here
12-28-13 edit taken directly from the article
" After a few fires and explosions, Perkins modified his system by installing a temperature-limiting device, operating the system at a lower pressure and temperature. The Perkins’ system was very popular in England until the 1850s but then died out. The idea of high-temperature hot water systems languished until they were revived in the 1920s."
The article goes on to state how the system grew in popularity
I would presume that old tried and true technology would work now. I think that with a little research, since these systems really need to be designed by an engineer to get the flow right, a slightly more efficient system could be had.