This is the best overview of spaced repetition research I have seen online or offline. It's thorough, well-argued and probably covers more than you were looking for. It's not specifically about Chi... Read more.

gwern.net

HiNative is a Q&A site where you can ask people all over the world about culture, language, anything. We support over 100 languages. Read more.

hinative.com

Duanmu San's "The Phonology of Standard Chinese" is by far the best introduction to Mandarin phonology that I'm aware of. It's mostly useful for people who like phonology or are already at an advan... Read more.

hackingchinese.com

This is a unique resource that shows how most common speech sounds are actually produced. Most other resources use animations and diagrams, but this is the frist I see which show actual MRI scans. ... Read more.

seeingspeech.ac.uk

This is a presentation by Thomas S. Mullaney about Chinese typewriters through history. In the digital age, typing in Chinese is easier than it has ever been, but how did the Chinese typewrite evol... Read more.

youtu.be

Character frequencies for Classical Chinese (i.e. not based on texts). This resource was discussed in this article: [The most common Chinese words, characters and components for language learner... Read more.

lingua.mtsu.edu