Traceur (http://code.google.com/p/traceur-compiler/): Its basically JavaScript with proposed language features for ECMAScript Harmony (http://wiki.ecmascript.org/doku.php?id=harmony:proposals).

To add to this, each language has an about link and an engine link (shown at the bottom when you select a language). These will take you to a page describing the language and the interpreter we're using, respectively.