EaseChinese: Provides a large collection of reviews and recommendations of Chinese language learning resources, and more.
Learn Mandarin Chinese Online: Free Mandarin lessons on grammar and vocabulary.
Min Multimedia: This site offers simplified Chinese tutorial lessons, with hyperlinked characters and flashcards.
Pinyin.info: A Guide to the Writing of Mandarin Chinese in Romanization: This site presents rules for using pinyin and other romanization systems, as well as non-romanization systems. It also contains related tools, books and links.
Similarities Between Tongyong Pinyin and Hanyu Pinyin: Comparisons at the Syllable and Word Levels: This research paper discusses similarities between tongyong pinyin and hanyu pinyin at the syllable and word levels.
Sinoling.com: This site offers a variety of Mandarin Chinese language resources, categorized into linguistics, links, culture, and reference. There is also an interactive forum.
XueZhongWen.com - Your online Chinese buddy: This is an online portal and forum for students of Mandarin Chinese at all levels. Topics range from listening, grammar and idioms to Chinese history and culture.