Exercism
On #Racket, samth linked to Exercism, a wonderful analog to $LANG Koans. Each exercise is a basic, small program which you code by satisfying the unit tests. Unlike, say, Clojure Koans, the Clojure Track of Exercism has you create 58 programs of various degrees of complexity, rather than a running through individual test assertions.
You code enough to pass the tests, then submit. You can keep tweaking your solution, and keep submitting it. On your dashboard, you’ll be able to see how your solution has evolved over time. You can also look at how other people solved the problem, discuss solutions, etc.
Exercism, with its emphasis on creating programs and then discussing them, is an even better way to introduce a language than a standard koan runner. Especially considering that if you already know the language, instead of solving the exercises you can easily submit new ones for others to solve.