hackattic - No nonsense, real world programming challenges
Indie Hackers - Connect with fellow entrepreneurs, developers, and bootstrappers who are sharing the strategies and revenue numbers behind their companies.
Codewars - Achieve code mastery through challenge.
Case Study Club - Learn how people design digital products
Real Dev - Real-life coding skill challenge
WorksHub - Helping Software Engineers find paid work, learn and get hired through Open Source contributions. WorksHub is made up of computer scientists, machine learning experts and the best in talent acquisition.