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HTML Dog has been dishing out healthy code treats since 2003. The idea is, and has always been, to take the somewhat convoluted official specifications of client-side technologies (HTML and its relations) and present them in a much more readable, easier to understand, fashion.
CodingBat is a free site of live coding problems to build coding skill in Java, and now in Python 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.