User code execution platforms

At the moment, the only platforms known to support serializeable continuations and sandboxing at the same time are Lua/Pluto and the java Rhino interpreter for Javascript.

PyPy (unforuntely) doesn't seem to be there quite yet, based on my discussions on pypy-dev.
http://www.mail-archive.com/pypy-dev@codespeak.net/msg05398.html
http://www.mail-archive.com/pypy-dev@codespeak.net/msg05419.html

Every result for NodeJS + Continuations indicates it has no support for them. Apparently the underlying V8 engine only has rudimentary, platform-dependent support.
https://groups.google.com/group/nodejs-dev/browse_thread/thread/aadbb0354e7cad31
http://comments.gmane.org/gmane.comp.lang.javascript.nodejs/17757

Rhino

http://www.mozilla.org/rhino/

Sandboxing Rhino
http://stackoverflow.com/questions/93911/how-can-you-run-javascript-using-rhino-for-java-in-a-sandbox
http://codeutopia.net/blog/2009/01/02/sandboxing-rhino-in-java/

Rhino Continuations API for Java (1.7 R2)
http://blog.norrisboyd.com/2009/03/rhino-17-r2-released.html

Rhino Continuations
http://wiki.apache.org/cocoon/RhinoWithContinuations
http://helma.org/wiki/Continuations/#RhinoContinuations

Rhino Scopes and Contexts
https://developer.mozilla.org/En/Rhino_documentation/Scopes_and_Contexts

Rhino 1.6r2 included with Java 6
http://en.wikipedia.org/wiki/Rhino_(JavaScript_engine)

Lua

Lua is a multi-paradigm language

Designed to allow flexible semantics.
Which implements coroutines nicely
Supports flexible sandboxing, although not RAM/CPU limiting.
Has the Pluto Library for serialization of coroutines/threads.
Can be embedded from other languages. Is another language even required?
Has the Orbit MVC web framework
Can be self-parsing
Metalua should provide a backend for graphical editing of code

NodeJS

http://nodejs.org/
Does not support serializeable continuations, probably do to limited V8 support.

Jefe, sandboxing for NodeJS. Uses a process pool, allows CPU/RAM, and execution time limiting.
https://github.com/fictorial/jefe

JSApp.US - Example of sandboxed NodeJS execution, but uses another sandboxing solution
http://jsapp.us/

And yet another NodeJS sandbox, but only supports timeouts
https://github.com/dominictarr/node-sandbox

NodeJS framework:
http://geddyjs.org/

And a long list of cool javscript modules

Javascript rewriting solutions (CPS instead of callbacks)

These do not offer continuations or serialization directly, only a (sometimes) generic solution for rewriting code to have callbacks (NarrativeJS and StratifiedJS).
StreamlineJS, on the other hand, adds functions which rewrite (at runtime) only the code they are applied to into CPS form.