 <p>Probably the most secure so far is <a href="http://www.scala-js-fiddle.com/">http://www.scala-js-fiddle.com/</a> (<a href="https://github.com/lihaoyi/scala-js-fiddle">code on GitHub</a>) simply because it does not even run the code on the server, but on the client!</p> <p>The gotcha is: it's not truly Scala code, it is <a href="http://www.scala-js.org/">Scala.js</a>, which is a dialect of Scala, is still experimental, etc. But it <em>might</em> be enough for your use case.</p>