MessageBox.Show(webBrowser1.Document.Window.Frames["main"].Document.Body.InnerHtml);

HTMLDocument doc = (HTMLDocument)webBrowser1.Document.DomDocument;
object j;
for (int i = 0; i < doc.parentWindow.frames.length; i++)
{
   j = i;
   HTMLWindow2Class frame = doc.parentWindow.frames.item(ref j) as HTMLWindow2Class;
   if (frame.name == "main")
    {         
       MessageBox.Show(frame.document.title);          
    }
}  
 

IHTMLDocument2 doc = (IHTMLDocument2)webBrowser1.Document.Window.Frames["main"].Document.DomDocument;

private void webBrowser1_Navigating(object sender, WebBrowserNavigatingEventArgs e)
{
   string url = webBrowser1.Document.Window.Frames["main"].Document.ActiveElement.GetAttribute("src");
}