nciku - Chinese-English dictionary with handwriting recognition, lots of examples, a text-to-speech system, and other learning materials.
Rikai.com - A web-mediator that adds mouseover pinyin readings and English meanings to Chinese webpages.
WrittenChinese.Com Free Chinese-English dictionary. Search results are ranked by frequency of occurrence in everyday Chinese text.
Zhongwen.com - A Chinese character dictionary and cultural reference. Characters are represented by images, meaning they are still viewable without Chinese fonts. Detailed character trees.
Mandarin Play - Learn Mandarin Chinese with simple and fun mini-games. The games combine English, Pinyin, Hanzi and audio to help you learn beginner and intermediate Mandarin Chinese in an interactive way.
ChinesePod - Free Chinese podcasts for beginners on various topics.
The Chinese Outpost - Site focused on Chinese language learning and culture, centered around an "Introduction to Mandarin Chinese" with readable and entertaining units on pronunciation, characters and grammar.
LearnchineseOK.com - Web's biggest directory of free online resources for learning Mandarin Chinese language
Mnemosyne - Free, reliable flashcard program. Has special "vocabulary" cards.