Quick Summary:
We have a completed flash game similar to Tetris that is ready to be \"linked up\" for multiplayer. After doing some research and asking here: Flash Sock
Full Disclosure: I am a cofounder of the union platform.
if you want to get up and running quickly, i would suggest using an existing multiuser server and client framework, such as the ones suggested already or union platform (www.unionplatform.com, free for 1000 simultaneous connections).
people tend to underestimate the amount of infrastructure work required to make a full-featured multiplayer game with reliable lobby systems and match making. if you are building from scratch, you might spend as long on, say, connection failover and reconnection code as you do on core game physics.
in a game like multiplayer tetris, i would implement authoritative game/world logic on the server (in union, you'd use a room module), and mirror the server-side simulation on the client side. the client shows output, interpolates the server-side world, and provides player inputs. here's an example multiplayer pong game that uses that approach:
http://www.unionplatform.com/?page_id=1229
one potentially useful feature of union for your situation is its support for javascript websocket in addition to traditional http communication.
http://www.unionplatform.com/?page_id=1587
the javascript support gives you the option to create full-fledged game clients in html5, or add native web browser clients for displays statistics or spectating games being played in flash. union's protocol is also publicly documented (http://www.unionplatform.com/?page_id=86), so you can build custom client features in any language.
out of curiosity, do you have a public link for your game?
have fun!
colin
I have used Flash Media Server to develop a number of multi-user apps (including games). Personally I think it's too pricey for what it does for you.
I've never used Red5 or Node.js, but the socket connections from Node.js sound promising.
I've also built a simple never-ending-socket using PHP which works as well (but not as good, it's a little jumpy) I bet you could find more info on that and perhaps develop something yourselves seeing as how you won't have to transfer too much data between users. On the web there's a number of examples of using PHP sockets with *SWF*s like this chat room example on kirupa.com
Best of luck.
I've had some success with SmartFoxServer. It's free for under 100 concurrent users, but if you have more you need to buy a license.
Still it's easy to write the server side logic and it's easy to manage and deploy.