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 hosts an excellent blog with a large number of insightful articles into Chinese characters. Some of them are directly useful because they talk about learning characters, others ... Read more.

outlier-linguistics.com

This site (originally a book) explains the difference between commonly mixed up (traditional) characters, published by Taiwan's Ministry of Education. As it says in the introduction, the main purpo... Read more.

language.moe.gov.tw

This is a Q&A page where teachers answer questions about Mandarin, mainly vocabulary use and grammar, but sometimes other questions as well. It's mostly useful because of the wealth of information ... Read more.

hwjyw.com

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

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

This blog provides in-depth explanations of some grammar and vocabulary topics in Mandarin Chinese. While there aren't a huge number of articles so far, there are enough to issue a general recommen... Read more.

mandarinfriend.wordpress.com