Also, I have enjoyed the Clojure koans as well: https://github.com/functional-koans/clojure-koans
Once you solve a problem you get a link to the solutions, but if you hit "Back" you end up back on the problem anew, with no link to move on.
Takes some tab management to keep the "next problem" link and see the other solutions.
http://www.pythonchallenge.com/
upon a cursory look I couldn't find a decent version for ruby, but there are many out there that do it differently.