OpenStreetMap creates and provides free geographic data such as street maps to anyone who wants them. The project was started because most maps you think of as free actually have legal or technical restrictions on their use, holding back people from using them in creative, productive or unexpected ways.
I found this link to maps of China including some of the maps already mentioned elsewhere, but some of them I've never seen before so maybe I'm not the only one.
Collection of China maps:
Especially the historical maps seems very interesting:
I have not looked at all of them but some of them have Great Wall information.
A very good map of the Shanhaiguan area, which seems to be an old US Army map from 1944 but orginally from 1927. Interesting to see how area was before they build the dams in the area:
This is an old German map of the Beijing area from 1875 also showing some Great Wall (Grosse Mauer):
Map of whole Ming Changcheng:
Map of Great Wall starting from the north:
Ming Changcheng in Liaoning:
Ming Changcheng in Tianjin, Beijing, Hebei:
These are the maps I used for my half-thru-hike in 2007. Those are Russian military maps, resolution is 1:500.000 and conver whole China.
In Germany they can be bought on CD as well as printed.
Their accuracy is _very_ high, the data is _very_ reliable. They offer an extremely usable grid for GPS navigation.
Drawback: The Chinese names are written in cyrillic. You should be able to read and pronounce them and then cross check to chinese maps and everything will be fine.
The CDs can be bought in Germany for about 75 EUR each (aprox. 115 USD).