Parallel.js: A javascript library for parallel computing (adambom.github.com)

Doesn't look like it. Clicking "Try It" does nothing, so I guess it has no single-threaded fallback.

You need a way to spawn threads. There may be a way to implement this in the main browser event loop but I have yet to think of a way.

Anything that needs weak references. They might land in ES7, though...

Check out the github repo. I wrote a worker.js "polyfill" that's meant to emulate web workers for node. I want to apply this to the browser at some point but for now there are hard dependencies on Worker and URL.