I pointed out a website dealing with Japanese history below, and now I'd like to point one out dealing with the history of China.
The China History Forum is outstanding. It's HUGE. I only wish I had the time and intellectual bandwidth to explore it in depth. There is a wealth of very knowledgable people posting on every conceivable topic on the history of China.
It is very high quality and well worth visiting. You can reach the China History Forum by clicking on the title of this post, or by following the link over at the right.