I've been using Red5 Flash Streaming Server for a while and the more I use it, the better I like it.  Sure, Adobe Flash Media Server is an excellent commercial product which does the same thing and supports server-side ActionScript.  Red5 doesn't support server-side ActionScript at the moment (this will probably change), but it is an open source Flash server and with a little bit of Java knowledge you can achieve the same result and possibly more.

The following example shows how to display the total number of subscribers that are currently connected to this simple Flex application.  If you open this blog page in multiple browser windows, the number should increase. The reverse will happen when you close browser windows.  This will obviously also depend on the number of other clients connected to the app, but because this is such a "high traffic" blog you probably won't have to worry about that :)  Many thanks go to Learnosity for providing me with Red5 hosting, which makes this demo work.

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If you are interested, take a look at Red5 server documentation.