Tae Kim's work is probably the equivalent for Japanese [0]. I think his "complete guide" is still a work in progress, but his grammar guide is pretty much the defacto free resource.
I actually suggest imabi.net over Tae Kim depending on how you want to define "learning the language".
Tae Kim is a great introductory guide to Japanese and is something I often hand to beginners. But for anyone who wants to seriously study the language, imabi.net is on an entirely different level.
[0]: http://www.guidetojapanese.org/learn/