There are several socket servers on the market that fit the development of Flash virtual world. Some are good at media streaming while some are powerful on user management.
SmartFoxServer (http://smartfoxserver.com/) was developed by gotoAndPlay(). It is a socket server for Flash that provides powerful tools and resource management to enable highly productive development of virtual worlds and multiplayer games.
It was designed for Flash originally and now it also provides a set of API for iPhone, .Net, Java, Silverlight, and even Ajax. It also supports media streaming function by embedded open source Flash media server called Red5.
It also supports clustering through terracotta, an open source clustering ...