Zak allows you to select one or more MP3 files and convert them into a single MP3, MP4, M4A, or M4B file. The primary use for Zak is audio books. Most audio books are distributed in several files, usually one per chapter. Converting these files into a single .m4b allows one to take advantage of the iPod's bookmarking features.
zgsmplay is a curses-based realtime GSM file player with file selector. (GSM is a lossy sound compression format used for speech.) In addition to simply playing the files, zgsmplay lets you move around them, etc., and can cope with large samples. A copy of Jutta Degener and Carsten Bormann's free GSM compressor/library is included.
zlplay is an MP3 player that uses the zlcatalog system to organize songs. zlcatalog is a library for organizing things in non-hierarchical ways. Songs in zlplay can belong to multiple groups. Navigation through the groups shows songs and related groups. So, for example, it's very easy to start listening to all of your songs randomly and then when you hear a song by a particular artist, a single click switches the song list to songs by that artist.