Microsoft has launched a stand alone client (download here) for their recently updated Games for Windows Live service. The client features a Marketplace, Media and Overview section; but what's surprisingly missing is anyway to browse achievements, view your friends list, or send and receive messages. Granted this is only the first release of the client, but right now it's little more than a glorified webrowser into the marketplace. I know personally the marketplace is the last of those listed features I wanted from a standalone client.
Perhaps soon we'll see an events channel similar to the NXE? (like the subtle segue?)