JQuery creator John Resig has been participating in the ECMAScript 4 efforts, and in a recent blog post reviews the current implementations of the spec.
Predictably, the ECMAScript 4 reference implementation provides the most new features, followed closely by Tamarin, a join VM project between Mozilla and Adobe. Included in the list is the Mbedthis commercial implementation focused on embedded Web servers, as is Mozilla's Spidermonkey, and the Java-based ECMAScript implementation, Rhino.
Resig notes that,
I went through and attempted to compile as many bugs and features as possible, as stated by the ECMAScript 4 specification and double-check them against all the actively-maintained implementations... I think it's fascinating to note that there [are] 3 implementations that already have over 25% of all the new features in the language implemented.
If you've looked at the ECMAScript 4 language, what new language features do you like the most?