Caja and Narcissus are definitely comparable parallels, but Continuum is unique in that it fully implements the ECMAScript object model, runtime, and standard library (except for a few things which are still in progress). I would say that Continuum is the only one that qualifies to be called a "JavaScript engine" in its own right.

Narcissus is a meta-circular interpreter that puts a thin layer over the host engine, and requires that host engine itself to have some ES6 features (it can only actually run in Spidermonkey and V8 with the --harmony flag enabled).

Caja is more of a wrapper that protects access to capabilities than anything. In fact it specifically avoids fully parsing source code itself in order to be performant. Its goal is to sandbox code, not interpret it.

Continuum implements the ES6 Object Model and nearly fully implements the internal algorithms of the ~450 page ES6 specification (aside a small handful of things which are a bit out of date or remain to be implemented). In fact, the only thing it does not implement itself is RegExp, which it currently wraps the host engine's functionality to provide. It even fully implements the Date internal algorithms (https://github.com/Benvie/continuum/blob/gh-pages/engine/bui...).

Additionally, it self-hosts much of its own code. All of the ES6 standard library is itself written in ES6 (https://github.com/Benvie/continuum/tree/gh-pages/engine/bui...) and is executed in the virtual machine each time a realm is created. Roughly 25% of Continuum's code is written in ES6, while the other 75% is written in ES3 (I plan to reverse this ratio in time).