While I’m sure there are a lot of similar resources that deserve equal mention, I wanted to post a link while it is fresh on my mind. There are a great collection of online maps of China available for direct viewing via the website of the University of Texas Perry-Castañeda Library Map Collection. They include a number of contemporary maps and many maps from earlier in the 20th century, in particular, especially from the U.S. Army Map Service. The only criticism I have is that the date of the maps is not always clear.
See also its wonderful Japan topographical and city plan maps from the US army occupation period.
If you agree that this is a great resource, consider leaving them a comment thanking them for making it available online via the U of Texas library comment page.