string html = string.Empty;
Uri uri = new Uri(@"http://www.tuosito.it");
System.Net.WebRequest request = System.Net.WebRequest.Create(uri);
using (System.Net.WebResponse response = request.GetResponse())
{
  using (System.IO.Stream stream = response.GetResponseStream())
  {
    using (System.IO.StreamReader sr = new System.IO.StreamReader(stream))
    {
      html = sr.ReadToEnd();
    }
  }
}