Codewell.cc - Improve your HTML and CSS skills by practicing on real world design templates.
codebunk - Realtime Collaborative Editor with Code Execution, REPLs and Chat
Good Code - Free front end coding challenges
Floobits - Floobits brings real-time collaborative editing to text editors, IDEs, and now Atom.
One Degree - The Human Search Engine
coderpad - Collaborative code editor with in-browser, real-time execution. Conduct programming phone screens like a boss.