Japanese
Jisho.org - This is the best online Japanese dictionary ever. I've also used it for looking up traditional Chinese characters. You can look up words, phrases, sentences, etc. If you need a dictionary for Japanese, look no further.
JapanesePod101 - This website was an invaluable resource to me when I was first starting out. They have podcasts, PDFs, quizzes, and videos. I got a subscription that I used constantly during the first two years of my Japanese study, and learned the bulk of my basic conversation skills from them. They have material for all levels, so definitely check them out and try a subscription!
Tofugu - The link will bring to you a page on how to determine kanji stroke order. Tofugu is one of the biggest resources I used when I first started learning Japanese. He has a YouTube channel as well. He not only teaches and speaks in Japanese to help you with listening and reading, but he goes to Japan and vlogs about his adventures.
Books I used - This is one of my old YouTube videos I made before I went to study abroad at Ritsumeikan, so I have a lot more I use now. However, if you are just starting out, this video might help you a lot.
日本人の知らない日本語 - This book was intended for Japanese people (who don't always use their own language correctly) and it covers cultural aspects, keigo, kanji, etc. I think it's a funny series, and some of the books have quizzes and short essays. Good for reading practice.
日本語総まとめ for JLPT - I bought two of these for kanji and reading comprehension in order to study for the N1. Many of my friends swear by these books. I like the reading comprehension one a little more than the Kanji one. These books are set up in the JLPT format so it gives you a good idea of how to study for the test, but much extra practice is needed to memorize kanji readings and getting a good grammar explanation book is advised as well.For learning Japanese culture, Japanese dramas, TV programs, and NHK World are all good, but obviously you will never know the culture until you come here and enjoy it for yourself!
Korean
Talk To Me in Korean is my favorite resource for learning the Korean language because they have EVERYTHING from learning phrases that come up in Kpop music, dramas, etc., but they also have grammar, vocabulary, YouTube videos, and best of all - it's all free! You can listen to a lesson, download the PDF, buy their textbooks, and be speaking Korean in minutes. I visited the You Are Here Cafe in Seoul and it was really great to be able to use my Korean to order a smoothie and then I went to the Self-Study room where Kyung Hwa was helping out people who had Korean questions!
How to Study Korean - This is a very comprehensive site that covers grammar, vocabulary, etc. The site is organized so that you can follow along in order, but you can always search for grammar points you are having trouble with. They also feature short stories written in Korean for learners who want an extra challenge in reading comprehension.
Hanguk Drama and Korean - I love Shanna. She is so honest and helpful when it comes to finding good Korean study resources. I found the textbooks I'm using now (the Ewha Korean series) through her review. She also studies Japanese.
Eat Your Kimchi - Simon and Martina do a fantastic job of giving you the lowdown on Korean culture and occasionally teach you Korean words that are popular/useful. They talk about Kpop, fashion, culture shock, and FOOD. They collaborated with TTMIK to open the You Are Here cafe in Hongdae, so you could say they are pretty big in Korea. They've also met many Kpop stars and have been on Korean TV. (They now live in Japan and do videos on Japanese culture.)
Seoulistic - This site is mostly for people traveling to South Korea, but they give a ton of advice on where to shop, where to eat, and cultural mistakes to avoid. Definitely check them out if you are planning on visiting in the future or even if you already live in Korea and are strapped for something to do.
Korean Comics - This is a short webtoon done in Korean (though you can read it with English translations at the bottom) that is cute and fairly easy to understand. There were only a few things I had to look up, but I learned a lot reading this, and it was fun to understand most of it. Little stories like this are great to boost your confidence and show you how far you have come in your language ability!
Naver Comics - Naver has lots of free Korean comics, and the artwork is stunning most of the time. I love to try to decipher what is going on, and since I'm not yet in Korea and I can't buy actual manhwa or things to read all in Korean, this is a great free resource the let me challenge my grammar and vocab ability. This post on webtoons is also really good to get you started on Korean comics.
Dramafever - You know I had to do this. I started learning Korean because I wanted to watch Korean dramas without subtitles (and usually did anyway if a new episode wasn't subbed yet). You do have to pay to get rid of ads, but they have pretty much every drama you've ever dreamed of. I recommend starting with Coffee Prince, My Love from Another Star, or something historical like Faith. Pretty much anything with Lee Minho (The Heirs) or Jang Geun Seok (You're Beautiful) is excellent.
Korean Idioms and Proverbs - This is a nice free resource for Korean idioms and their meanings in Korean and English. I know there are books written on this sort of thing, so it's good I found a way to study idioms for free!
Fresh Korean - A blog with lots of downloadable pdfs and other Korean resources.
Children's Books in Korean (downloadable) - A small sample of children's stories in Korean, scanned from books and in pdf form. Categorized by difficulty.
Korean Literature in Translation - A wonderfully detailed blog featuring book, story, and poetry reviews. You can find works that are translated into English or the original Korean works themselves. I'd recommend starting with this post on getting started with Korean literature.
Korean Poetry in Translation - A blog dedicated to Korean poetry (includes original language and English translations).
German
Duolingo - I've been using this for a while and I really love it for German. I've used it for Spanish, French, Japanese, and Korean, but I think Japanese and Korean aren't as suited for Duolingo as the others I've tried. I love the Stories feature and the fact that the streak helps me stay motivated to practice every day!
DeepL - Amazing translator app for text! It lets you select specific words to find out what part of speech they are, and it also gives you examples of the word used in different sentences.
PONS - This is a translator and dictionary site/app that works really well. Very easy to use and helpful for learning words in context.
DW.com - Interactive stories/videos that quiz you on what you hear or see. Great for learning in-context German. They have different level courses and it works on both mobile and desktop.
German.net - A helpful resource for reading comprehension. It gives you short readings and quizzes you on the information. Good for beginners - not as much advanced content. They also have vocabulary and grammar sections to help you learn.Coffee Break German - A podcast with a German native speaker and an English native speaker (Scottish) talking about phrases, grammar, and culture. They do grammar sections as well as culture sections about Germany. Easy to listen to and starts from the very beginning. Found wherever you download your podcasts.
Dino lernt Deutsch - A fun way to learn German - through reading an actual book! Each chapter is a story about Dino's new life in Germany. Difficult vocabulary are bolded and defined at the end of each chapter. Good for even beginners who want a sense of accomplishment.
Mandarin
MindSnacks - I love this app and it's great to go back, even after almost a year without studying Mandarin, and see the progress I made and how much Chinese I can still remember. There are many different languages you can get this app for, such as German and Portuguese, so go look if they have what you want to learn! It teaches you through games and tracks what skills you are best at.
Chinese Reading Practice - I love this site because it teaches you through conversation and stories. When I was studying Chinese, I learned through Traditional characters, so I would put these stories (simplified) into Google Translate and translate them to traditional.Again, watching Taiwanese dramas was the biggest reason I started learning Mandarin, and it really helped with my listening and speaking skills. Don't just listen. Repeat. Try to copy what the actors are saying and you will learn a lot faster that way.
Other
Memrise - I stumbled upon this app recently and I LOVE it. There is an app version and a website version and I have it on all of my devices. It really is the best way to learn vocab. They have ways to create your own or use other user's mnemonics, and there are vocab lists from everything. I've been using the Talk To Me in Korean Intermediate level vocab lists, as well as lists for JLPT N1 kanji. They don't just have languages; they offer any subject you need to memorize things for, so definitely check them out.
Fluentin3Months - Benny is such an inspiration. He doesn't let himself get frustrated and give up. He knows the secrets to language learning and is willing to share his progress every step of the way. He is constantly learning new languages, so go check him out for any advice/resources he provides.
TED talk on Language Learning - I love this talk. I agree with him completely. If you want to learn a language, you have to first get your mind into gear and believe that it's possible. Don't get discouraged and work at your own pace, but challenge yourself to go beyond what you thought was possible.