Data maps (Tufte, 1983) represent one or more attributes associated with areas of a map. See for instance the map in Figure representing the cancer mortality in the USA at Lung, Trachea, Bronchus and Pleura for white males in years 1970 - 1994 in counties. Data maps can carry a huge volume of data in small space, but carry an intrinsic problem: they may give a wrong illusion that the importance of the information is geographic, rather than factual (Tufte, 1983). For instance, the figure doesn't allow ordering the counties according to the number of cancer deaths, or the number of people living in the county.
Another technique based on distorsion is used in http://www.worldmapper.org. Worldmapper is a collection of world maps, where territories are re-sized on each map according to the subject of interest.