Hello, fellow developers!I am Mai Trần Tuấn Kiệt, as an web developer I know how hard it is to constantly learn and improve. And how easy it is to jump over some underrated knowledge that is necessary to become a great developer.During my journey, I found resources that are so valuable that I really can't believe they are free. Here are two of them:1. Refactoring Guru:This site has a ton of knowledge, full of beautiful Illustrations and clear explanations that I haven't found any better.It covers two major topics in detail: the art of refactoring and design patterns.You will find out how to determine exactly why your code is dirty and the correct methods to clean and make your code readable and easier to maintain.And also, there you can read about all the 22 classic design patterns.Oh, and it has code examples in multiple languages!2. Exercism:If you are learning or thinking of learning a new programming language, this is the right spot.Exercism is a platform to learn by practice, through exercises from the hello world to advanced and hard challenges.There is more than 70 languages to learn, and some of them have special roadmaps to learn. Where all exercises are grouped in topics, and you need to learn a topic to unlock others that build on top. This is great to learn fast and not loose time jumping from article to article, going back and forth.Some languages and techs have great docs and teach you well, others not so well. So Exercism is always a valid thing to add up to your learning experience.And, that's all for this article. Thank you for reading, and don't forget to contribute and let down in the comments a resource you didn't believe is free or that cheap for how great it is.