try another way to Do.parallel, inspired by Tim Caswell's howtonode blog articles

another wait.js

function Wait(fns, done) {
    var count = 0;
    var results = [];
    this.getCallback = function(index) {
        count++;
        return (function(waitback) {
            return function() {
                var i=0,args=[];
                for(;i<arguments.length;i++) {
                    args.push(arguments[i]);
                }
                args.push(waitback);
                fns[index].apply(this, args);
            };
        })(function(result) {
            results.push(result);
            if(--count == 0) {
                done(results);
            }
        });
    }
}
var a = new Wait(
	[
		function(waitback){
			console.log('done a');
			var result = 500;
			waitback(result)
		},
		function(waitback){
			console.log('done b');
			var result = 1000;
			waitback(result)
		},
		function(waitback){
			console.log('done c');
			var result = 1500;
			waitback(result)
		}
	],
	function(results){
		var ret = 0, i=0;
		for(; i<results.length; i++) {
			ret += results[i];
		}
		console.log('done all. result: '+ret);
	}
);
var callbacks = [a.getCallback(0),a.getCallback(1),a.getCallback(0),a.getCallback(2)];
setTimeout(callbacks[0], 500);
setTimeout(callbacks[1], 1000);
setTimeout(callbacks[2], 1500);
setTimeout(callbacks[3], 2000);