There are a number of sites offering free Japanese lessons, but they're of varying quality, and none are complete. Here are the ones we rate as the best.
You need to register for Japanese Online but it's free, and you gain access to a series of lessons intended to start from complete beginner level. In reality, it would probably be too fast if used as an independent course - particularly because the Japanese Hiragana script is introduced with inadequate explanation. It's not used at all in the first lessons, so get going with those first, but when it is, there's no gradual explanation. At that point go to one of the sites listed below which deal with written Japanese. In any case, it's probably the best free site on the net. Each lesson starts with a dialogue which is available in written form with the translation and the audio. An explanation of the grammar and vocabulary contained in the dialogue is then followed by a couple of brief exercises. In addition, you can also access a selection of grammar and multimedia activities.
To supplement Japanese Online, try eLanguageSchool or Study Japanese, both of which are good on explanations of both the Hiragana and Katakana scripts and the grammar but suffer from lack of audio and no or insufficient practice activities. The same is true of Free Japanese Lessons and Teach Yourself Japanese.
Lack of audio is again the problem with what would otherwise be a very useful site : Nihongo o narau - Learn Japanese. But again, its explanations and exercises are a useful supplement to Japanese Online. it also has some fun sections. While you're there, check out the lyrics of Rudolph The Red Nosed Reindeer in Japanese.
Japanese has three writing systems : Hiragana and Katakana - which are the phonetic systems taught in the courses listed above, and Kanji - a set of over 7,000 ideographic characters imported from China. About.com has a good explanation of the differences and which are most useful. If you want to learn Kanji, then check out the University of Chicago's site Kanji Alive, and later test yourself with Drill the Kanji.
The Japanese Grammar Course also has a guide to the three writing systems of Japanese, and then grammar explanations going up to a good intermediate level. Again no audio and no practice material, but again useful in conjunction with other courses.
If you're at a more advanced level, test your understanding with a series of Dictation Quizzes
And if you're a teacher or home-schooling parent looking for tips on methodology, lesson plans and practical activities to use in the classroom, go to our section for Teachers and Parents.