gtk123 is a frontend for mpg123, mpg321, and other mp3 players that use the same command-line format. It should be possible to use gtk123 in situations where most other MP3 players would be ineffective due to lack of CPU power. gtk123 is being developed and tested on a 486DX4/75 laptop. On such a system, it can be somewhat unresponsive when decoding VBR, but CBR works well (at least in mono mode).
skiTunes is a fast, extensible alternative to iTunes. It can use Apple Quicktime, VLC, MPlayer, LAME, and cdrecord. It differs from Apple's offering in that it is written in Cocoa and uses native widgets, as well as supporting formats iTunes doesn't, such as OGG and WMA. Using native widgets will allow it to automatically match the rest of the OS when a theme is applied, unlike iTunes, which requires hacking in order to make it match. It will also offer a skinnable "mini- mode".
qplay is a console music player frontend (for mpg123, mpg321, ogg123, etc.) that features a user-friendly inferface for managing a play- and queue-list and a module-like system for adding new file formats. It uses familiar vim key bindings. The playlist can be played in sequence or in shuffle mode while an extra queuelist has highest priority so that you can determine which song will be played next before returning back to sequence or shuffle mode. It also features efficient sort and randomization, custom key bindings, external control support, and support for MP3, Ogg, M3U, and FTP streams.