This list is constantly evolving and is supposed to help you find great learning material to improve your German. The order is not based on quality, I merely aim to provide a good list of sources that you can use.
Lingolia
This is an amazing platform to learn about most of the important grammar topics in the German language. They also provide exercises and material to practice your German in each topic. The free version is absolutely enough, but they also have a paid version which provides you with more exercise material. I can really recommend this website.
German.net
Here you can find a lot of amazing material that is for free. Also, they have tons of exercises that you can use. The website might not win a design award, but it is a powerful tool to get your German to the next level. They also indicate if an exercise is easy or more advanced which helps you to measure your progress. They also have a great section on verb conjugation. All in all, a great free resource to learn German.
lingua
On this website you can find good souces to practice your reading an listening, among other things. What I also like is that we website offers the descriptions and guides in many languages. So when you learn German, but don’t speak english very well then you can also choose your native language. I use this especially with my beginner student to practive pronounciation.
yourdailygerman
This is a beautiful website with daily topics to improve your German. The author uses funny memes to make his point about the grammar topic and the content is really useful stuff, even I as a German teacher us it to look up some of the nuances in the German Grammar. This site is a well of knowledge especially if you speak already some German. I can highly recommend it.
Learn German with anja
THE youtube channel to learn German. It’s pretty fun too. Anja focuses on teaching everyday German and does that in her unique way. When you watch her videos you feel so motivated to learn German. She also has a website where she offers a course which many people use. Learn with her seems like a lot of fun.
Expertlygerman podcast
Yes, I know, this is my podcast. But I really think it is a great podcast to improve your German. Especially for beginner, intermediate and advanced level German speakers. What is cool about the podcast is that you can listen to me teaching someone else German and you can also listen to episodes that are more advanced news topics in German.
This podcast is, I believe, really the best way to learn German.
Some more helpful websites:
German Online gym
This is a great and concise course for beginner level speakers. Great if you want to start out.
Deutsch-podcast
Another great podcast to improve your German. Listen to the two hosts who do a fabulous job in teaching you German.
Easy Deutsch
They have easy explanations, grammar lists, exercises and much more.
grammatisch
A great app to learn German!