Sorry to necro a very old thread, but folks looking for tutorials/sample code for multiplayer games can find info on creating chat, lobbies, game rooms, logins, and specific games such as pong and fridge magnets here:
The examples run on the Union Platform, a development platform for creating multiuser applications. The Union Platform includes the Union Server, a multiuser communications server, and Reactor, an ActionScript 3.0 framework for creating Adobe Flash client applications.
Union has an entirely free licence for 1000-users for both commercial and noncommercial projects. It also has a live test server anyone can use to get started without installing any applications or writing any server-side code.
Hope that helps!
(ps: as a historical note, "Unity", which was mentioned earlier in this thread, evolved into the Union Platform.)