Learn Traditional and Simplified Chinese Characters online. Basic membership is free.

Online mandarin courses, chinese dictionary, chinese calligraphy studio, chinese name translation, chinese songs, chinese poems, chinese editor, chinese annotation tool, chinese dictionary, converters for pinyin, unicode, traditional and simplified chinese, tools to build chinese websites, etc...

Taiwan's largest discussion forum in English - talk about living... working... staying fit... eating well in Taiwan (and more). Established in 1998

A Chinese learning podcast- both the podcasts and the written versions of the lessons are available for free.

Free online dictionary with useful additional tools like a Chinese text annotator.

Online Chinese-English dictionary. Also has Chinese to French, German, Italian, Spanish, Portuguese and Russian.

A very comprehensive source of information about pinyin and the romanization of Mandarin Chinese.

Tool for converting tone numbers to tone marks in Hanyu Pinyin. For example, Han4yu3 Pin1yin1 => Hànyŭ Pīnyīn.

Includes some very useful advice about learning pronunciation and tones. There are also Chinese study book reviews.

World Learner Chinese is a learning platform for the online training of Mandarin Chinese. WLC offer on-line interactive Chinese courses  and live on-line language instruction for private, corporate and group training. World Learner Chinese: Where people of the world meet.

Chinese-English dictionary from Yahoo Taiwan. Entering Chinese characters outputs the definition in English and Zhuyin Fuhao (bo po mo fo) and Pinyin to indicate the pronunciation. There is also audio for the pronunciation.

Zhongwen.com contains the complete text of Chinese Characters: A Genealogy and Dictionary
using the character tree system. It is a great resource for learning about and understanding Chinese characters.