jeKai Japanese-English dictionary - An ongoing project to build a free, open Japanese-English dictionary with explanatory definitions, illustrations, and examples.
Jim Breen's WWWJDIC Server - This server requires a browser with a Japanese environment. (It should enable your browser to detect the coding automatically, but if you are having problems seeing the Japanese, it may be best if you set your browser to EUC-JP mode.) If you are using Netscape or IE on a PC, can I recommend HongBo Ni's excellent NJWIN as a method of displaying Japanese characters.
Kanji Dictionaries for Japanese Learners - Sites on dictionaries for learning kanji published by the Kanji Dictionary Publishing Society, especially New Japanese-English Character Dictionary (Kenkyusha and NTC) and The Kodansha Kanji Learner's Dictionary.
Kiki's Kanji Dictionary - Online, browsable Japanese Kanji dictionary. Providing over 3,000 kanji are explained in Japanese and English along with sample compounds, and over 19,366 unique examples of kanji compounds.