Multiplayer games can be created using xml Sockets. It's probably the best way of doing things, and most 'live'.
Do a search on google.com for 'aquaserver xml socket server' for a good free version. If you require anything with more users, or more control, check out http://www.flashnexus.com
Nexus has a lot of server side functions which add alot more security, and less flash programming.
We've also a downloadable chat example, and a demo of 4-player bomberman!