Zhongwen.com is a great
interactive dictionary of characters. The flexible organization of
characters used on the site makes it incredibly easy to look up an
unfamiliar character! (Uses traditional character images and pinyin.)
Popup Chinese has a
variety of free tools and Mandarin lesson recordings. Subscribe
for transcripts and other materials.
Pinyin.info has a lot of
information about pinyin.
Chinese tools provides
some other useful tools for working with Chinese characters.
The CEDICT
Chinese dictionary is used on this site to provide
interactive definitions of words.
The Unihan
Database was used to create this website. It is a very useful
database of Mandarin, Cantonese, Japanese and Korean characters,
linking the Unicode to BIG5 and GB2312 character sets, and giving the
pin1yin1 pronunciations for every character.
Pinyin Joe is a great
reference for setting up your computer for Chinese and pinyin.
Memrise is an impressive
site for studying various languages including Mandarin. The site
generates flashcard-like tests on the fly, using your own study
history to psychologically optimize the tests for fast learning.