4clojure was launched recently. It takes a lot of inspiration from Functional Koans, and several of the puzzles from 99 Lisp Problems; but it improves on both by providing a learning experience for which you need only your browser.
Problems vary in difficulty from language tutorials like What is the second element of this list? to thought-provoking questions like Is it possible to arrange these N words so that each differs from the preceding one by at most one character, and more problems are being added every day.