parallel.js

var Parallel = exports.Parallel = function Parallel(actions) {
  this.actions = {};
  this._emitter = new process.EventEmitter();

  if ('object' === typeof actions) {
    var keys = Object.keys(actions);
    for (var i = 0, key; key = keys[i++]; ) {
      this.add(key, actions[key]);
    }
  }

  return this;
};

Parallel.prototype.add = function add(name, action) {
  this.actions[name] = action;

  return this;
};

Parallel.prototype.bind = function bind() {
  var callback = arguments[arguments.length - 1],
      names = Array.prototype.slice.call(arguments, 0, -1);

  for (var i = 0, name; name = names[i++]; ) {
    this._emitter.addListener(name, callback);
  }

  return this;
};

Parallel.prototype.run = function run(callback) {
  var count = 0,
      action,
      args,
      keys = Object.keys(this.actions),
      self = this;

  for (var i = 0, key; key = keys[i++]; ) {
    action = this.actions[key][0];
    args = this.actions[key].slice(1);
    args.push((function(name, action) {
      return function() {
        arguments = Array.prototype.slice.call(arguments);
        arguments.unshift(name);
        self._emitter.emit.apply(self._emitter, arguments);
        callback.apply(self, arguments);

        count--;
        if (0 >= count) {
          callback.call(self, null);
        }
      };
    })(key, action));

    count++;
    action.apply(this, args);
  }

  if (0 >= count) {
    callback(null);
  }

  return this;
};

usage.js

var Parallel = require('./parallel'),
    fs = require('fs');

var task = new Parallel({
  one: [fs.stat, '/etc/passwd'],
  two: [fs.stat, '/hello/world']
}).
  bind('one', 'two', function(err, stat) {
    // Do something with both tasks
  }).
  bind('one', function(err, passwdStat) {
    // Do something with passwd stat
  }).
  run(function(name, err, stat) {
    if (null === name) {
      // Everything done!
    } else if ('two' === name) {
      // Handle hello/world
      stat;
    }
  });