Jisho - recommended online Japanese dictionary, takes English, Romaji, Japanese to find words and Kanji (also has a radicals search engine for finding the meaning of Kanji)
Kuma Sensei's Japanese Road Map! - excellent guidelines for navigating yourself to fluency in Japanese. Includes textbook links and Japanese Graded Reader (books to practice reading) links.
Imabi - super useful website outlining Japanese grammar (even includes classical and Okinawan Japanese!)
JLPT Guidelines - need to know for understanding which exam you should take
Rikaikun - Chrome extension that allows you to hover over Japanese text on websites to find the definitions of words, Kanji pronunciation, etc., much better than Google Translate!
Vocabulary and Kanji:
Anki - this application is amazing for vocabulary practice (not just in Japanese), but it has public decks that you can download for personal use (no need to make your own) and utilizes the spaced repetition phenomenon to assist in retaining words for long-term memory - can find tutorials on YouTube for understanding the program
Heisig's Remembering the Kanji - great resource for helping to memorize all 2,000+ common Japanese Kanji Characters
Wanikani - costs money for premium, but excellent tool to learn Kanji through spaced repetition and fun to use!
Commonly Used Vocabulary Per JLPT Exam - helpful for making flashcards for exam prep!
Memrise - free, similar to Quizlet, but has designed courses for languages like Japanese
Reading Practice:
NHK Easy - read Japanese news articles with furigana and simplistic language, perfect for learning new political vocabulary and general reading practice
List of Websites for Reading Practice - arranged by language level
Tadoku - company that created Japanese Graded Readers (good for beginner readers), sells books here, but also has other free reading resources
Japanese Stories for Language Learners (ショートストーリーて学ぶ日本語) - for intermediate readers, bilingual text that promotes language learning, includes voice over of the text as a CD
Parallel Text Short Stories in Japanese (日本語の短編小説) - for more advanced learners (3000+), great resource to transitioning into reading Japanese literature while learning the language (bilingual text)
Breaking into Japanese Literature - for more advanced learners, teaches you the styles of Japanese literature to prepare to read original Japanese text
Listening Practice:
YouTube Channels - list of 500+ YouTube channels that are perfect for immersion practice and learning Japanese through listening, ranges from popular channels like Kizuna Ai and Dogen to smaller channels owned by Japanese natives