SHOUTcast
SHOUTcast is a free solution for live and on demand audio broadcasting across the Internet or any other network.
For listening to SHOUTcast broadcasts a player compatible with streaming MP3 audio is required (such as Winamp for Windows, Audion for Mac and XMMS for Linux).
In order to broadcast you will need a streaming server, Winamp, the SHOUTcast Source Plug-in for Winamp and Fraunhoffer IIS MPEG Layer-3 Codec (available for free inside NetShow Services from Microsoft).
All of the required tools are available for free download here.