String site = textBox1.Text;
            String resp = null;
            HttpWebRequest WebRequest = (HttpWebRequest)HttpWebRequest.Create(site);
            WebRequest.Method = "GET";
            HttpWebResponse myWebResponse = (HttpWebResponse)WebRequest.GetResponse();
            StreamReader Source = new StreamReader(myWebResponse.GetResponseStream());
            resp = Source.ReadToEnd();
            myWebResponse.Close();
            MessageBox.Show(resp,"Código-fonte",MessageBoxButtons.OK);