I know that's what you're talking about, but it would have helped if you said what you're listening to the HTTP packets with. CherryPy, for example, makes this very easy for you. The POST/GET arguments come in directly as arguments to the functions you map to pages.
http://cherrypy.org can teach you all about CherryPy.
Raw Sockets you would have to parse yourself. Not sure about CGI. Not aware of any other methods, other then the HTTP module that interfaces to the socket library.