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Mark Miller, the creator of E, is on the ECMAScript committee, and has been working very hard to ensure that ECMAScript 5/Strict is almost capability-safe, or, as he calls it "trivially securable".
He is currently working on SES (Secure ECMAScript), which is a capability-safe subset of ECMAScript, which, interestingly, can actually be implemented within ECMAScript 5/Strict itself. It basically consists of a small bootstrap script, which redefines some global objects and functions. Any code that is loaded after this bootstrap script has run, is guaranteed to be capability-safe.
They also have a superset of SES, called Dr. SES (Distributed Resilient Secure ECMAScript), which basically extends the capability-safety of SES into a distributed setting across multiple machines.
Previously, Mark Miller created Caja. However, being based on ECMAScript 3, Caja is much more complex than SES or Dr. SES, because ES3 is not "trivially securable". In particular, Caja is a pretty complex compiler toolchain, which performs a whole-program-transformation and some quite sophisticated static analysis.
Google caja is a pretty serious attempt: http://code.google.com/p/google-caja/