Jim Breen's WWWJDIC Japanese-English Dictionary Server : Possibly the most useful online Japanese dictionary. Gives kanji and romaji readings. Lots of example sentences. Very easy to use and well organized
Jeffrey's Japanese<->English Dictionary Server : Allows you to access different specialty dictionaries such as Science terms, Legal terms, Buddhist terms,etc.
Nihongo Resources : Various elementary to intermediate grammar resources and online dictionary
Charles Kelly's Online Japanese Language Study Materials : Lots of Kana, Newspaper, Kanji, Flashcards, JLPT resources, ect. **Check his LINKS page!!!**
Learn the Japanese Kanji : Need JAVA. Online Kanji flashcards. Organized by level. You can practice the On, Kun, and English meanings.
The Kanji site : Another online Kanji flashcard site.
Kanji-step : Features online JLPT practice tests.
AIEJ - Association of International Education Japan : Information on the JLPT, and Ryugakushiken
Pera-pera Penguin's 5-Minutes Japanese Class : Useful little lessons on a variety of topics by Hitomi Hirayama found in the Daily Yomiuri. Dowloadable PDF files. Updated every 5 weeks.
ALC online : (Japanese) The publishers of Nihongo Journel. Has some online articles from new issues and excerpts from past issues. If you are thinking of picking up the magazine, go here to learn more. You can order online if you live in Japan. If not you can go to Amazon and order it from their subscriptions list.
Tuttle Publishing : Famous publisher of Japanese language and culture books.
Kodansha International's The Japan Page : Japanese language and culture books.