Heads up! This blog post hasn't been updated in over 2 years. CodePen is an ever changing place, so if this post references features, you're probably better off checking the docs. Get in touch with support if you have further questions.

Wes Bos has a new totally free training course called JavaScript30:

30 Day Vanilla JS Coding Challenge
Build 30 things in 30 days with 30 tutorials
No Frameworks × No Compilers × No Libraries × No Boilerplate

We’re seeing a good amount of people tackle the challenges here on CodePen!