Exercism offers free coding exercises in over 60 programming languages, but the real difference is the mentoring. After you submit a solution, a volunteer mentor reviews your code and gives feedback on style, readability, and approach. That feedback loop is something most online courses do not provide. The exercises progress from simple to challenging and cover language features and idioms.

You can work through a track at your own pace, and the CLI downloads exercises to your machine so you use your own editor. The community is friendly and the mentors are generally experienced developers. For someone learning a second or third language, it is an effective way to write idiomatic code rather than just functional code.