hackattic - No nonsense, real world programming challenges
codebunk - Realtime Collaborative Editor with Code Execution, REPLs and Chat
Frontloops - Level up your frontend skills with handcrafted tasks
Floobits - Floobits brings real-time collaborative editing to text editors, IDEs, and now Atom.
Daily Coding Problem - Get exceptionally good at coding interviews
coderpad - Collaborative code editor with in-browser, real-time execution. Conduct programming phone screens like a boss.