InCompletionOrder allows you to do foreach on collection of Tasks that will process them as fast as they complete.

AsyncExtensions.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Threading;
using System.Threading.Tasks;

namespace Asyncfun
{
    public static class AsyncExtensions
    {
        public static IEnumerable<Task<T>> InCompletionOrder<T>(this IEnumerable<Task<T>> tasks)
        {
            var inputs = tasks.ToList();
            var results = inputs.Select(i => new TaskCompletionSource<T>()).ToList();

            int index = -1;
            foreach (var task in inputs)
            {
                task.ContinueWith((t, state) =>
                {
                    var nextResult = results[Interlocked.Increment(ref index)];
                    nextResult.TrySetResult(t.Result);
                }, TaskContinuationOptions.ExecuteSynchronously);
            }

            return results.Select(r => r.Task);
        }
    }
}

Program.cs

using System;
using System.Linq;
using System.Net;

namespace Asyncfun
{
    class Program
    {
        static void Main(string[] args)
        {
            PrintSites();

            Console.ReadLine();
        }

        private static async void PrintSites()
        {
            var sites = new[] { "https://vimeo.com/68390480", "http://www.google.pl", "https://twitter.com/", "http://www.microsoft.com", "http://mateusz.swietlicki.net", "http://www.google.com" };

            var tasks = sites.Select(
                async site =>
                    new
                    {
                        Url = site,
                        Source = await new WebClient().DownloadStringTaskAsync(new Uri(site))
                    }).ToArray();

            foreach (var task in tasks.InCompletionOrder())
            {
                var site = await task;

                Console.WriteLine(site.Url);
            }
        }
    }
}