YellowBridge: this is a both a dictionary and a learning resource for students at all levels. It includes free flashcards, HSK prep materials, reading materials and tools, and more. Most of the features are free, although it has ads and offers a paid subscription. Simplified and traditional characters.
ImproveMandarin: a website with tons of linguistic and cultural resources for learning Mandarin, as well as many guides and tips for taking the HSK. There are also twenty-six lessons for beginners that can help self-studiers with the fundamentals of the language. Simplified characters.
Centre for Teaching Chinese as a Foreign Language: an archive created by the University of Oxford of resources from their Chinese courses. There are many helpful pages for grammar, vocabulary, pronunciation, test preparation, and more. Simplified characters.
Hacking Chinese: a blog with tons of tips and tricks to improve your Chinese. It is less of a resource teaching Chinese and more of a guide on how to organize your study habits and methods. Many resources on the website are free, but the courses and other pages are behind a paywall. Along with their own content, they have a list of other online resources here.
OpenLearn: some free online Chinese courses for beginners. Simplified characters.
ChineseTools courses: another source for free online courses and tutorials with audio. Simplified characters.
MandarinX: a program created by EdX that provides online, self-guided Chinese courses. However, you only have access to the materials for a short time and don't get all of the added features unless you pay. Simplified characters.
Coursera: a learning platform similar to EdX that also provides some free courses in Mandarin. Simplified characters.
Writing Chinese characters
ArchChinese: a learning resource specifically for learning to read and write Chinese characters. There are several games and reference resources, as well as a built-in dictionary to look up stroke order.
汉字笔顺: This website can show you the stroke order of any simplified or traditional character by searching them in the search box in the upper right-hand corner.
Memrise: a flashcard-based learning app with tons of community-made study sets to learn Chinese. There is a "official" course developed by Memrise in Simplified Chinese, community created modules, and HSK/TOCFL preparation materials. You can also find modules to learn Cantonese, Hakka and other dialects.
Chinese-Tools.com: this website has tons of themed vocabulary lists and interesting resources for learning Chinese. Simplified characters.
Chinese Vocabulary Wiki: this is a wiki with vocabulary points separated into difficulty levels. A really helpful feature is that it has articles comparing similar/commonly confused words. Simplified characters.
Chinese Grammar Wiki: This is a great reference website for questions about Chinese grammar. Grammar points are separated by difficulty/HSK level, and you can also use the search function to look up words or concepts that are difficult for learners (like 把 structure, 了, verb complements, etc.). Simplified characters.
Chinese Measure Word List: This page lists 115 commonly used measure words and includes an explanation of measure words in Chinese. The page is most helpful at the bottom because it separates them into different categories. It's a good reference material, but if you are a beginner the information may be overwhelming. Simplified characters with some traditional characters.
Chinese Reading Practice: A website with many articles separated by beginner, intermediate and advanced (based on the old structure of the HSK exam). Simplified characters.
HSK Reading: A similar website to Chinese Reading practice, but these articles have a few comprehension questions at the end. Simplified characters
ZBSchools: this is a Singaporean website that is publishes short articles for children, but they are also be helpful for non-native speakers. When reading, readers can choose to show Pinyin or use a build-in dictionary and also filter between articles, news, comics, and more. Many articles are free, but some are behind a paywall. Simplified characters.
LangCorrect: a community where you can have your foreign language writing corrected by native speakers by posting it online (it requires making an account). It is not just for Chinese and can be used for any language.
HiNative: a language learning community with a question-and-answer format. You can ask questions about the vocabulary, grammar, or culture and be answered by native speakers. In order to post questions you must make an account.
Chinese Text Project: this is a free and open-source resource to read famous pre-modern Chinese texts. This may be useful for students studying Classical Chinese. It has tools to see Pinyin, word meanings, and some English translations.
Glossika: Glossika is a paid resource that uses AI technology and spaced repetition to help users memorize common phrases. For some threatened and endangered languages, the service is completely free. The Chinese dialects available for free include Hakka (Hailu and Sixian dialects), Taiwanese Hokkien, and Wenzhounese. Mandarin and Cantonese are available with a subscription.
Omniglot also has a great list of online tools for learning Mandarin, Cantonese, and other dialects of Chinese.