List of free preparation stuff for developerππ½
If you're looking for all programming tools around preparation (not just free). Check this page List of preparation Programming Tools

LeetCode
Level up your coding skills and quickly land a job. This is the best place to expand your knowledge and get prepared for your next interview.
Tag: learning-resources, skill-test, preparation
Programming challenges from beginner to expert Interview questions

HackerRank
HackerRank is the market-leading technical assessment and remote interview solution for hiring developers.
Tag: learning-resources, preparation, recruitment
There are Free competitive programming challenges in Java, C++, PHP, Python, SQL, JavaScript, Algos, Data Structures and AI.