Red5 is a free, open source, Java based, alternative to Adobe’s Flash Media Server. Altough the server is in pre-alpha there are many production-ready applications that work on Red5 and many have chosen to use Red5 because its free and open source.
To install Red5 you need a dedicated server. You can not install Red5 on a shared hosting plan. Red5 is available for both Windows and Linux. You can find more info about it here: [http://osflash.org/red5 Update 8/10/2015: This no longer exists]
All tutorials and support material on the website will now be updated to include Red5 specific info.