Free code-practice tools or service for developers

CodeAbbey

Collection of programming problems to practice solving, learn to program and code, and win certificates

Tags: learning-resource, code-practice

Codeforces

Codeforces. Programming competitions and contests, programming community

Tags: algorithm, learning-resource, code-practice

Codewars

Codewars is a platform that helps you learn, train, and improve your coding skills by solving programming tasks

Tags: learning-resource, code-practice

Project Euler

Project Euler is a series of challenging mathematical/computer programming problems that will require more than just mathematical insights to solve

Tags: learning-resource, code-practice, algorithm