This site by Jun Da contains a lot of very interesting resources for anyone who wants dig deeper into frequency lists. One set of data I haven't found anywhere else is a frequency list of syllables... Read more.

lingua.mtsu.edu

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

Mandarin Chinese With Fiona Tian is a free video resource for learning Mandarin Chinese in a fun, enjoyable way. Chinese doesn't have to be hard. Kick back, relax, and enjoy. Read more.

mandarinmadeez.com

Finally, Duolingo added a En->ZH course. As Duolingo is the most popular gamification learning app, the learning experience is relatively streamlined and tested. The monetarization aspect leads to ... Read more.

en.duolingo.com

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

TutorMandarin is an online Chinese tutoring software and service. Students take 1-on-1 classes with professional Mandarin tutors on PC, Android, or iOS. Full courses from Beginner to Advanced, Spok... Read more.

tutormandarin.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

From their website (if you have personal experience of their courses, please leave a comment): New Concept Mandarin specializes in teaching Mandarin Chinese (Putonghua) as a second language. Act... Read more.

newconceptmandarin.com

Subtitles offer a way to get endless annotated Chinese audio for free. You use linked-up learning! This article on Chinese Boost includes some resources for people who are looking for subtitles in ... Read more.

chineseboost.com