When Real announced they had open-sourced their server software, I wasn't all that excited -- all the cool proprietary tech is binary-only. But the client they are now releasing has a bunch of cool infrastructure implemented (including the Real Time Transport Protocol suite of protocols, alphablending, volume management, proxy auto-discovery, and more). It will ship with MP3 and H.263 support, and Ogg Vorbis support is coming. So it's useful out-of-the-box.
It's dual-licensed, and the "open" license has not yet been approved by the Open Source Initiative.