You can use WebKit.NET
Look here for official tutorials
This can not only grab the source, but also process javascript through the pageload event.
webKitBrowser1.Navigate(MyURL)
Then, handle the DocumentCompleted event, and:
private documentContent = webKitBrowser1.DocumentText
Edit – This might be the better open source WebKit option: http://code.google.com/p/open-webkit-sharp/