• CIA World Factbook
  Useful info on the world's countries, population, maps, etc. All the site and the illustrations it uses are public domain.
• Copyright Law in Japan
  In English and Japanese.
• 40 Maps that Explain the World Part 1 Part 2

• Crayola Ideas Website
  Ideas for things to do with elementary children.
• Yahoo! Games for Japanese Kids
  Interactive online lessons for Japanese children on various subjects.
• Jeopardy Labs
  Create a Jeopardy game, or browse ready-made ones.

• Irasutoya
  Ever wondered where all these cute illustrations came from? This is the answer.
• Undraw
  Open source illustrations. You can specify an accent colour, so it goes pretty well with presentations.
• illust AC
  Free to use illustrations (5 per day with a free account).
• Isometric
  Isometric illustration. It looks professional, but has a bit of a "startup" vibe.
• Fresh Folk
  Cute and fresh illustrations. Needs an email and then you can download the whole library (spam free).
• Humaaans
  Same as the above.
• pngimg
  Plenty of illustrations with transparent backgrounds. Great for montages. Goes really well with GIMP.
• Glaze
  Free to use as long as you credit the source. You only get access to a png.

• Drawkit
  Hand-drawn vector illustrations and icon resources.
• manypixels
  Offers svg, and you can even customize the accent colour.
• Ouch
  Free vector and 3D illustrations.
• Absurd design
  I can't explain why, but I love these illustrations.
• Mega doodles pack, by MariaLetta
  Cool, cute, open-source.
• Open Doodles
  A free set of open-source illustrations.
• Illlustrations
  100 illustrations designed as part of a challenge. All free to use.
• LukaszAdam
  Free SVG illustrations, you can download the whole pack for free.

• Canva
  Great webapp for creating professional looking flyers/posters (you can apply for a free account as a non-profit organisation).
• Slides Carnival
  Free Powerpoint templates.

• Pixabay
  Royalty-free photographs and images. Don't hesitate to take a look!
• Unsplash
  Community-driven library of free to use images. You can also contribute your own shots ;)
• Pexels
  Same as above.
• The noun Project
  Free pictures and icons. Great resource
• photo AC
  Free to use stock photos.

• The noun Project
  Free icons and pictures. Great resource.
• Flaticon
  A library of free-to-use icons.
• Iconmonstr
• Icomoon
• Orion icon library
• Material icon
• Game-icons
• heroicons
• Zondicons

• Colorkit
  A free online tool to find palettes, gradients, for all your graphical needs.
• Colormind
  A similar tool that uses AI and where you can see previews for websites.
• Adobe color
  This one is neat to find some colours based on colour harmony.
• Palettte (with three t's)
  An advanced tool to create your palettes. May not be suited to beginners.

Colour theory
These tutorials can help you better understand and use colour.

• The HSB Color System
  A bit of theory on colour on screen.
• Ultimate Guide to Color
  There you have it, the ultimate one.
• Colour Method
  A little game about matching colours, it's fun, but useless, but fun.

• GIMP
  A free image manipulation software. You can do basic editing, using calques, transparency layers, crop/resize images.
• Krita
  A painting program, more akin to Photoshop than GIMP. It offers some more advanced features, and is also geared towards digital painting.

• Inkscape
  Is to vector graphics what Gimp is to raster graphics. Very nice, but has somewhat of a learning curve.

• Darktable
  A photo developping/editing tool. It has similar capabilities to Lightroom. It's profiled denoizing tool is pretty amazing.
• Rawtherapee
  Another photo editing tool. It's a bit more intuitive to use than Darktable, and also produces good results.

• 10ten Japanese Reader
  Open-source Successor to Rikaichamp, it lets you hover over a word and get a pop-up dictionary. For Firefox, Chrome, Edge, and Safari.

• Eijirou
  Very good online dictionary with tons of example sentences.
• Jim Breen's Online Japanese Dictionary
  Excellent, open source, plenty of different Japanese dictionaries.
• Linguee
  Full of real examples of translations.
• Jisho
  One of the most popular among Japanese learners. Based on JMdict.
• Weblio
  Good dictionary with decent example sentences. Japanese monolingual also exists (see below).
• Weblio Thesaurus
  Great for finding synonyms.

Important: data safety cannot be guaranteed when using these tools, so I recommend not feeding them sensitive/personal info.

• Deep-L
  Based on Linguee, it is often pretty accurate.
• Excite
  A Japanese online translation tool in 7 languages.
• Google Translate
  Infamous online translator.

General
This is not a joke. Usually a good place to start.

• Expressions
  This site is very old and it shows. But you have more than 1500 idioms and expressions in both languages.
• Japanese Expressions
  A selection of 500 Japanese expressions and their English translation.
• Sound effects translations
  An online dictionary for Japanese-to-English SFX translations.
• Manga/Movie/Region/Chemistry/Accounting/Abbreviation
  This is a niche one but it is useful for translating movie or manga names.
• Linguistic terms
  Might help when you want to give or understand grammar explanations.
• Haiku
  Haiku topical dictionary based on the Nyūmon Saijiki.
• Shinmei Database
  Not a glossary, but a list of gods that appear in the Kojiki with detailed information and further sources for more reading.

• Japanese Law Translation
  Very specific but darn accurate. The rest of the website is super useful as well.
• Contracts
  Terminology that shows up in contracts.
• Practical Patent Terms Dictionary
  This one is extremely user-unfriendly, but I include it here for completeness sake.
• Japanese-English Glossary on Local Governments
  A free online version of the eponimous book.
• Facilities
  Vehicles/Facilities that you might find in a city.
  

If you need any of these resources, all my prayers to you.

• Finance Terminology
  A neat site by Mizuho with tons of explanations for Japanese words related to finance.
• Accounting
  As the name suggests.
• Tax
  This one is self-explanatory too.

• Medical
  Lots of medical terms, very niche but might prove useful.
• Pharmacology
  Japanese Pharmacological Society's glossary of pharmacological terms.
• Kusuri no shiori
  A database where you can look up information about medication. They offer both an English and Japanese version.

• Sushi and fish names
  If you ever have to translate a sushi place's menu.
• Nautical terms
  Again, I do not know if you'll need it but it's here.
• Culinary terms
• Onsen

• Digital Dictionary of Buddhism
  You'll probably never need this one, but hey if you ever do... it's here. Login with ‘guest’ as a username.
• Jōdo-shū Buddhist Terminology
  Obscure Buddhist words in romaji, kanji, and English (internet archive)
• Glossary of Japanese Buddhism (Wikipedia)
• CJKV-English Dictionary of Confucian, Daoist, and Intellectual Historical Terms
  Similar to the first one, and edited by the same person, but you also have historical terms.

• Japanese Post Zipcode
  A neat way to get the reading for any place you might not know. Just enter the Kanji in 市区町村・町名を入力.

• Jisho by Monokakido
  The best dictionnary app, unfortunately for iOS and MacOS only. You can buy dozens of monolingual/JP-EN dictionaries and the app is super easy to use. I cannot recommend this enough.
• Kotobank
• Weblio
• Slang dictionary
• Kanji jiten online
• Contemporary Japanese Corpus
  This is not really a dictionary but a corpus of contemporary written Japanese. You type a word and it shows you in which context it likely appears (what particles it can take, etc.). really useful when you have a word but are unsured how it's used.

• Japanese.io
  Great tool for reading Japanese. You can import your own texts into it.
• NHK News Easy
  Aimed at an around N3 level, it's good to learn vocabulary to talk about recent events.
• Syosetsu
  A community-driven website to post/read novels in Japanese.
• Aozora Bunko
  Here you can read all books that are in the public domain. So it's mostly older books, but it's free.
• UVA Japanese Text Initiative
  The University of Virginia Library and the University of Pittsburgh East Asian Library's collaborative effort to make texts of classical Japanese literature available on the World Wide Web.
• Reajer
  A variety of bilingual texts to practice reading in Japanese.

• NHK for School
  Full of educative programms for Japanese children up until the end of middle school.
• こせんだ式日本語教室
  YouTube videos about learning Japanese, in Japanese. The guy is super nice.
• 日本語の森
  Same as above, but more popular. I'm more a fan of the people that used to run the channel back in 2017, but it's good nonetheless.
• SuperNative
  Trains your hearing with video clips. You need an account to use.
• Hirogaru
  A variety of videos/blog articles about different subjects related to Japan. By the Japan Foundation.
• Animelon
  A website where you can stream anime with subtitles in Japanese.

• Common Errors in English Usage
  By the Washington State University.
• English Stack Exchange forum
  Lots of questions about English usage and things even native speakers don't know.
• Japanese Style Sheet
  Monumenta Nipponica by Sophia University.
• The Elements of Style
  A concise guide to English style, rules of language usage, and principles of composition.
• UN Correspondance Manual
  Great guide for formal English writing.
• Grammar Girl
  Free, archived audio shows and complete transcripts from the Grammar Girl English podcasts.

• Mail/Letters
  Examples of mail/letters that you can write in Japanese. This is edited by the Japan Post.
• 文例書式
  Different examples of Japanese written documents.
• Wajutsu
  Several ways to express yourself in Japanese.
• 敬語の指針
  A really complete and thorough guide on keigo written in Japanese.
• 3秒敬語
  A ‘keigo machine translator,’ that lets you input plain Japanese and get keigo. There are also settings for more or less verbosity.
• Easy Japanese for Work
  A website by the NHK to learn some formal Japanese as well as business manners.

Flashcards

• Anki
  A neat little app where you can create your own decks of virtual cards to review vocab, or even download pre-made decks. Learn more about how to use it here.
• Quizlet
  Less intimidating than Anki, easier to use but also less customisable.

Nintendo 3DS

• 漢検トレーニング２
  A game centered about studying for the Kanji Kentei. I used it to get 191/200 on the pre-2. You can also get it on an emulator.

Nintendo Switch

• 図書館ⅡSW・名作１００選＋α
  A neat little soft with plenty of classic texts to read in Japanese for only 800 yen. There also are a few DLCs for even more reading material.
• 漢検スマート対策
  An official kanji-kentei app, with 5 years of past problems inside for all the levels. It´s a wonderful resource.

Language Exchange

• Tandem
  An app to find people to chat with in your target language.
• Hello talk
  A social network where you can exchange with people who want to learn your language.

Lessons

• Cafetalk
  A website to find tutors for mostly anything, including Japanese (1 on 1).
• Bright Start Japanese
  A business that offers personalized Japanese Lessons at various locations within Japan.

Miscellaneous

• Calil
  You can search for any Japanese book and verify its availability in Japanese libraries. Or you can search your local library and see what books they have.

• Anna's archive
  A neat website to download ebooks of old books that have fallen into the public domain. Please do not try to search for copyrighted material like Nihongo Sou Matome, Shin Kanzen Master, or『デキる人の敬語の正しい使い方』
• Itazuraneko
  They have a great library of Japanese books. I haven't read it, though.
• Nyaa
  A peer-to-peer website where you can share your non-copyrighted Japanese material with others.