Icecast is a free open source streaming media serverthat currently supports streaming of Vorbis, Opus, WebM and Theora. Also known to work are MP3 and AAC. Not every encoding is supported for every protocol (see Icecast on Wikipedia).
On Ubuntu, Icecast is provided by the icecast2 package.
Use this tag for questions about streaming with Icecast on Ubuntu. If your problem is recieving a stream that appears to be working for others, it's helpful to include other descriptive tags--in particular, a tag for whatever audio player you're trying to use for streaming.