Exemplo de como obter uma string com o HTML de um site específico.
HttpWebRequest request;
HttpWebResponse response;
Stream stream;
StreamReader streamReader;
string strSiteUrl = "http://cbsa.com.br";
request = (HttpWebRequest)WebRequest.Create(strSiteUrl);
response = (HttpWebResponse)request.GetResponse();
stream = response.GetResponseStream();
streamReader = new StreamReader(stream);
string stringHTML = streamReader.ReadToEnd();