There is a multitude of free Japanese learning resources available to everyone. Here are a few notable ones:
Japanese Learning Guides
This section discusses various guides to learning Japanese.
Really complete guide introducing many of the issues facing Japanese learners today and how to get around them. If you're going to read another guide, make it this one. Written by shoui520. In general the website is a gold mine and I highly encourage you to go through the various guides and resources hosted on it.
This 30 day routine is meant to introduce newer Japanese learners to the immersion method. This was also written by shoui who wrote the guide above.
A more concise guide explaining the basics of learning Japanese in a clear manner. Written by QM. The website in general is very useful and is recommended reading to everyone.
An older guide explaining the basics on learning Japanese, albeit slightly outdated in my opinion. The anki section especially shows its age and there are better guides for setting it up. Still, this is one of the earlier guides on the subject and it's still worth reading through.
A spaced-repetition system is a tool meant to help you remember things. It is used by medical school students, law students, language learners, geography enthusiasts and many more people. The main spaced-repetition system used in the Japanese learning community is Anki, though other have become popular over the years such as wanikani and JPDB especially. It is my belief that Anki with a heavily modified setup is still the best SRS we have available at the moment, although JPDB is a really good resource as well and I highly encourage you to check out both and then decide.
The main Anki website with download options. Please refer to one of the Anki guides below or mine before downloading and setting it up, as vanilla Anki has a lot of issues.
The official docs for Anki.
Another concise guide written by QM, this time tackling Anki and how to set it up correctly. Read also this guide discussing what type of cards should you add to your decks.
A very thorough guide to optimizing Anki discussing the various problems with vanilla Anki, most notably "ease hell".
A modern custom scheduler for Anki based on the Free Spaced-Repetition Scheduler algorithm.
Aquafina's various Japanese resources notably including custom handlebards for Anki and local audio. Various CSS options are explained in a clear and concise way with actual demos.
rudnam's mining and 漢検 Anki templates and the like.
This guide details how to setup an effective mining deck in Anki with Yomichan. Read this page as well to understand how to add media to your cards properly.
Another version of Core2.3k. By Infi who also made Yomibaba.
A popular mining setup (not mine, this is just the name of it). Thorough explanation of ShareX is a high point.
This guide was meant for people who don't want to fiddle with Anki and Yomichan themselves but instead want to use premade settings to get a mining deck set up fast. One notable highlight of the deck is the bilingual-monolingual dual setup in Yomichan and the media guides (manga, LNs, VNs). Written by Xelieu (xelieu on Discord).
Very convenient custom Yomichan CSS to color Yomichan dictionaries. Allows you to have different colors for different dictionaries.
An Anki deck discussing 四字熟語.
Eminent's note type for Anki. Really good although I prefer Aquafina's note personally.
A machine-translated (albeit by OPUS-MT) French version of JMdict. Might be useful alongside the original JMdict for French natives.
A great guide explaining the basics of CSS and HTML to help people create and style their Anki cards.
The best alternative to Anki I know and so much more. Very useful website, you can look up a visual novel (for instance) and get information about it (number of words, relative ease, etc).
Kana and Kanji
This section discusses Japanese scripts.
Ved's take on a game to help you learn kana. Great stuff.
Another cool kana game to memorize them.
Similar to the two above, a kana learning game.
A shortened version of RTK in anki deck format. Might or might not be useful to you, but worth trying out.
This video explains how to write all the kana. I don't recommend learning to write (especially kanji) until one is intermediate to advanced for various reasons but there's no harm in learning to write hiragana at least.
This guide compares various methods to learning how to write kanji. The best such guide I know. Written by QM.
A kanji quiz in website form.
A great Anki deck discussing phonetic components to help you guess and remember kanji readings.
A deck based on the book of the same name, based on the PhD dissertation of Natalie Hamilton.
This section discusses Japanese grammar and how to approach it.
A fantastic video series introducing Japanese grammar in a logical manner. Cure Dolly's speech can be bothersome to some so here is a transcript of what she is saying. RIP.
Tae Kim's grammar guide is a very well-known grammar guide in the Japanese learning community. Sadly, there are a few inconsistencies in the way he introduces important grammar points such as the copula or the "は vs. が" problem. Cure Dolly discusses this issue in this video. It is nonetheless good enough as a primer on Japanese grammar.
Another website discussing Japanese grammar. Very complete, even covers some Classical Japanese. Not as popular as the other two options because it is very large. My personal favorite introduction to Japanese grammar.
A very nice grammar reference master list. It is recommended you use your browser Find feature to locate the grammar item you would like help with.
Another very popular option to learn Japanese grammar. You can download the mp4 files to watch the lectures directly. The powerpoint slides are on the bottom of the website.
Amazing monolingual guide to Japanese grammar targeted towards intermediate-advanced learners that want to delve deeper into Japanese grammar than what Tae Kim or Cure Dolly provides.
Another great grammar resource written in Japanese that tackles the various JLPT grammar items. There is a corresponding anki deck that you can get here.
Anki deck discussing N1 and N2 grammar based on 日本語の森.
Anki grammar deck based on Bunpro.
A YouTube playlist explaining 国語, Japanese as taught in school in Japan.
Great explanations in Japanese on 国語 grammar.
This website introduces 国語 and the grammar used in the classics (古典).
This section discusses listening and understanding spoken Japanese.
An audio speech extractor to make for better passive listening immersion.
A tool made to help you recognize numbers in Japanese.
A website allowing you to search for Japanese sentences with native audio. The audio itself can be downloaded from here.
Great website discussing pitch accent with a handy list of resources. Highly recommended. Created by xythh.
Anime streaming website that lets you watch anime raw. Great way to ease into listening.
Another anime streaming website that lets you watch anime raw with Japanese subtitles and translation working directly in the media player for a seamless experience.
Website with some raw anime listed.
Browser-based media player made to allow easy Anki card creation with subtitles. Overall great media player for Japanese learners. Highly recommended.
Change the location to Japan and look for popular Japanese podcasts to listen to. I like this one.
A collection of general resources that do not necessarily fit in the categories above.
The resources section of The Moe Way. Very well classified.
Fantastic library, very complete with lots of resources from anki decks to visual novels.
The absolutely humongous resource section of Kuzuri's amazing website.
Tremendous browser-based reading app with tons of features for Japanese learners. Highly recommended for those interested in reading (light) novels.
Wonderful tool that lets you analyze text (for instance to check for frequency).
Really useful tool that uses an OCR tool to let you Yomichan (or select text from) pictures.
Image scans of various raw manga.
A really good anime upscaler. Requires a beefy setup though.
Great resource giving example sentences for various words.
Similar to yourei, lets you search for words and spits back example sentences.
Lets you search sentences from 青空文庫 works.
A very useful Japanese slang dictionary.
Cool onomatopoeia dictionary with pictures. Also check out the corresponding Yomichan dictionary.
The Japanese section of the free programming books GitHub repo. Now you can re-learn your favorite programming language in Japanese! I recommend Clojure.
A simple but fun kanji word game.
A 2D tower defense game that has you using kana and the like.
Absolutely crucial OCR app that takes manga and turns it into HTML files that let you parse the text with Yomichan.
Web page that helps you track manga reading progress using Mokuro. It requires using the author's Mokuro fork.
Application to OCR manga into PDF. Kind of like a graphical version of mokuro above but with PDFs.
Really cool locale emulator (as the name suggests). Lets you use applications with a Japanese locale without changing your system settings.
Great app that creates shortcuts for applications that will run through Locale Emulator above so you don't need to change your locale to Japanese to use the applications.
A pretty cool website that lets you see how Japanese characters interact with graphs.
Convert VobSub image subtitles into text subtitles.
Website with various Japanese subtitles. Click on the various links and you might find what you need.
Japanese color dictionary.
A website with various Japanese fonts on display.
A game hooker similar to Textractor. Very powerful and can hook games like Persona and the like.
This website has a bunch of Japanese dubbed movies.
A cool anime logging spreadsheet made by ねむい (nemuzora_nozomu on Discord).
An unofficial list of resources from the Languagegainz discord. Not officially endorsed by Aussieman (the creator of the Discord) but complete with links to various important Japanese resources.
A GitHub page listing various Japanese-learning resources and the like.