There are many resources to learn japanese.

General

• Jim Breen's site: A large and sophisticated collection of (free) resources, including dynamic dictionaries and translators

Kana

• ChipChat Japanese Training (free online applet to learn Hiragana & Katakana)
• Nihongo Resources (includes kana/kanji dictionary and lessons)
• KingKanji for learning kanji (commercial software)

Grammar and Words

• The Japanese Page (free lessons in grammar)
• Let's Learn Japanese (video & textbooks)
• Teach Yourself Japanese (Teach Yourself Japanese; Sounds, explanations, and a message board with lots of helpful people)
• Teachionary Japanese Word Sets play memory with sounds and words
• Japanese Lessons
• Reiko-chan's "Japanese for Anime Lovers"
• JWPce and JCE GNU-licensed dictionary and vocabulary trainer
• Teach yourself Japanese
• Nihongo Resources: Resource site including dictionary and Lessons
• Genki Online: Genki 1 & 2 Books, Excellent For Beginners.
• JLPT Kanji project has resources to study for the JLPT Japanese language test

Dictionaries

• Japanese Kanji–English Dictionary
• Japanese Katakana - English Dictionary