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.