var semaphor = new Semaphore(50, 50);
// We allow at most 50 threads for crawling
var resultPins = new List<Pin>();
// Results stored here
foreach (var pin in new HashSet<string>(pinIdList))
{
  semaphor.WaitOne();
  Console.Write(">");
  var pinClient = new WebClient();
  pinClient.DownloadStringCompleted += (sender, ex) =>
  {
   var html = ex.Result.Replace("\n", "");
   pinClient.Dispose();
   lock (pinIdList)
   {
     // do some post-processing and write back the results
   }
   Console.Write("<");
   semaphor.Release();
  };
  pinClient.DownloadStringAsync(new Uri(string.Format("http://pinterest.com/pin/{0}/", pin)));
}
for (int i = 0; i < 50; i++) semaphor.WaitOne();
// Wait until the last thread ends.
semaphor.Dispose();
Console.WriteLine();