The Salar ethnic group
Spanish language blog Chinochano has written a post about China’s 56 official ethnic minorities, and included scans from a set of stamps that features all 56 of them.
The post also links to two interesting resources: the website of Paul Noll, which includes a China page with various types of information from the meaning of colors in Peking Opera to statistics for computer ownership in China. There’s also a chart of China’s ethnic minorities with descriptions and population data. However it is not clear where much of this information is taken from.
Chinachano also links to the ethnic minority page on a site called Index-China, a comperehensive, illustrated list of the minorities.