The implementation includes releases for Windows, OSX (ppc), OSX (Intel), and Raw Source; written in the functional language, Standard ML, specifically using the Standard ML of New Jersey compiler.
If you haven’t been following the progress being made on the ECMAScript 4 language, now is a fantastic time to get on board. I’d recommend reading through the language documentation, subscribing to the mailing list, and following the implementation roadmap.
All of this should get you ready to play around with the reference implementation. Should you spot a bug, be sure to submit it to the bug tracker.