IceS is a source client for Icecast. It supports reading live from audio devices or stdin or rerecorded files from a playlist and send it to Icecast. Playlists can be updated on-the-fly. Also IceS is sometimes considered a reference implementation for Icecast source clients in general. It is maintained directly by the Icecast project.
Currently stable releases as well as ongoing development happens on version 2.x. Historically there was a version 0.x sending MP3 streams. This version has been deprecated and is not supported any more. Version 2.x does support sending streams in Ogg/Vorbis.
Stable releases can be found on the IceS Website.
For the latest version of ices0 have a look in the svn repository at
http://svn.xiph.org/icecast/trunk/ices0/. Note that this is only kept for reference.