CodingBat is a free site of live coding problems to build coding skill in Java, and now in Python (example problem), created by Nick Parlante who is computer science lecturer at Stanford. The coding problems give immediate feedback, so it's an opportunity to practice and solidify understanding of the concepts. The problems could be used as homework, or for self-study practice, or in a lab, or as live lecture examples. The problems, all listed off the CodingBat home, have low overhead: short problem statements (like an exam) and immediate feedback in the browser. The idea for CodingBat came from my experience teaching CS at Stanford combined with seeing how student's used unit-tests in more advanced courses, and crystalized when I saw an Owen Astrachan demo of a unit-testing thing he uses with his Duke students.