Slow Chinese - personal narratives by native Chinese people, read in Mandarin at a slow speed; topics vary from language, knowledge, tradition and culture to opinions on social phenomena
Popup Chinese - "modern and stylish" daily language podcasts
ChineseClass101 - audio and video lessons, flashcards, lesson notes, community forum
Chinese LingQ - Chinese conversations on a variety of topics
Ohio State University's National East Asian Languages Resource Center (NEALRC) provides access to learning materials for learners and teachers of East Asian languages.
Chinese: Communicating in the Culture, provided by NEALRC, is a curriculum for learning Chinese in cultural contexts, featuring textbooks, audio programs, and interactive multimedia.
Confucius Institute - live classes and self-learning affiliated with the Chinese Department of Education
Mango - offered free through Columbus Metropolitan Library. Get a CML library card from the CML website.
Hacking Chinese Resources - searchable database of hundreds of online Chinese learning resources; filter by level, topic, and resource type
Chinese Grammar Wiki - free online grammar resource with lessons organized by skill level
Tatoeba - database of example sentences searchable by language and word
Yoyo Chinese Pinyin Chart - pinyin chart with audio demonstrations and video explanations for English speakers
FluentU - Mandarin web video content with interactive Mandarin and English captions
Anki - free flashcard software (also available on iPhone/iPad and Android)