You can also invoke Web Services using HttpWebRequest class.

//Creating the Web Request.
HttpWebRequest httpWebRequest = HttpWebRequest.Create("http://localhost/DemoApp/Default.aspx") as HttpWebRequest;
//Specifing the Method
httpWebRequest.Method = "POST";
//Data to Post to the Page, itis key value pairs; separated by "&"
string data = "Username=username&password=password";
//Setting the content type, it is required, otherwise it will not work.
httpWebRequest.ContentType = "application/x-www-form-urlencoded";
//Getting the request stream and writing the post data
using (StreamWriter sw = new StreamWriter(httpWebRequest.GetRequestStream()))
{
    sw.Write(data);
}
//Getting the Respose and reading the result.
HttpWebResponse httpWebResponse = httpWebRequest.GetResponse() as HttpWebResponse;
using (StreamReader sr = new StreamReader(httpWebResponse.GetResponseStream()))
{
    MessageBox.Show(sr.ReadToEnd());
}

It will show some HTML content in the Message box.

You can also access or post data to .net web services. For sending data to webservices, you need to change the URL like following

http://www.dotnetthoughts.net/sample.asmx/HelloWorld.

where HelloWorld is the Web Method name to invoke. It will return an XML after successful operation.