Uri url=new Uri("http://aspnetmania.com");
Encoding e1;
HttpWebRequest myReq = (HttpWebRequest)WebRequest.Create(url);
myReq.UserAgent = "Mozilla/4.0 (compatible; MSIE 5.0; Windows 95; DigExt) ";
myReq.Accept = "image/gif, image/x-xbitmap, image/jpeg, image/pjpeg, application/vnd.ms-excel, application/msword, application/vnd.ms-powerpoint, *";
myReq.ContentType = "text/html";
myReq.AllowAutoRedirect = true;
myReq.MaximumAutomaticRedirections = 2;
HttpWebResponse myRes = (HttpWebResponse)myReq.GetResponse();
myReq.Credentials = CredentialCache.DefaultCredentials;
myReq.Timeout = 1000;
if (myRes.ContentType.Contains("text/html"))
{
e1 = Encoding.GetEncoding(myRes.CharacterSet);
Stream streamResponse = myRes.GetResponseStream();
StreamReader streamRead = new StreamReader(streamResponse, e1);
str = streamRead.ReadToEnd();
streamRead.Close();
streamResponse.Close();
}
myRes.Close();

И всё, в str будет храниться Html-код страницы.

Только не забудь str=Server.HtmlEncode(str);