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

Outlier Linguistics Dictionary of Chinese characters is an excellent tool to help you understand and thereby learn Chinese characters more effectively. It gives you instant access to the latest res... Read more.

outlier-linguistics.com

A blog written and managed by an advanced Chinese featuring reviews of books, TV dramas and games. Additionally, the blog also features recommendations for utilising media in Chinese language learn... Read more.

thecozystudy.com

This website allows you to look up all sorts of things related to characters, including pronunciation in historical and regional varieties, components based on their pictographic origin, and much m... Read more.

zi.tools

This is a digital version of the official authoritative stroke order standard used in China. It is sorted by stroke and is not searchable, so it's a bit hard to use. Thus, other resources will ofte... Read more.

yuke.blcu.edu.cn