This site contains a huge number of lessons, complete with texts, vocabulary, audio, exercises and much more. And it's all free. Note that if you want to get the intermediate and advanced material,... Read more.

collections.uiowa.edu

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

This website offers a large number of previous HSK and YCT Chinese proficiency tests, sorted by year and level. This is excellent for test preparation and/or benchmarking. Read more.

chinesetest.cn

Test your Mandarin Chinese vocabulary level in a few minutes. Get a an estimate of your HSK level, keep track of your progress to get to the next level! Read more.

hsklevel.com

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

This is a research paper detailing a study of the mutual intelligibility of Chinese topolects. Dialects were sorted into groups and then it was experimentally tested how much of words and sentences... Read more.

openaccess.leidenuniv.nl

This is a review of Perry Link's book about Chinese. This passage sums it up pretty well, and although I haven't read the book (yet), it's high on my list of books to get my hands on: "In writin... Read more.

lareviewofbooks.org