Example of creating a pool of Web Workers

WorkerPool.js

function WorkerPool(url) {
    this.url = url;
    this.pool = [];
}
WorkerPool.prototype.getWorker = function() {
    var w;
    if (this.pool.length > 0) {
        w = this.pool.pop();
    } else {
        w = new Worker(this.url);
    }
    return w;
}
WorkerPool.prototype.releaseWorker = function(w) {
    this.pool.push(w);
}

// Create a new pool
var my_pool = new WorkerPool("fib.js");

// Get a worker from the pool and use it
var worker = my_pool.getWorker();
worker.onmessage = function(e) {
    // Do stuff, then release worker if no longer working.
    my_pool.releaseWorker(worker);
}
worker.postMessage("start");

hey Andy!
did you find any library/solution which meets your requirements?
I also found your answer on stackoverflow and it looks like I'm looking for something similiar.

http://slowfrog.blogspot.de/2011/05/taking-advantage-of-multiple-cores-with_25.html
this article seems to be very helpful