List of Chinese/English names of Post Offices up to 1949 (on this site).
Very good for recent issues
USA based society. Many resources including Postal Rates in PRC and much more.
London, UK based society.
A wonderful specialist site for this area, including Manchurian Overprints section.
Specialist site for Liberated Areas, only North and North-East areas at present.
Part of the online WikiBook for all meter stamps.