Audacity is a cross-platform multitrack audio editor. It allows you to record sounds directly or to import Ogg, WAV, AIFF, AU, IRCAM, or MP3 files. It features a few simple effects, all of the editing features you should need, and unlimited undo. The audio I/O uses PortAudio, which fully supports OSS, Mac OS X CoreAudio, and Windows WMME, and can be compiled with support for ALSA and Jack.
Mr. Voice is a specific-purpose Perl/Tk front-end for an MP3 database. It was written to provide an easy-to-use MP3 solution for the production booth of an improv comedy troupe, and as such is tailored to providing quick searches and playback of MP3s that are split up into categories. While not intended to be a standalone desktop MP3 player, others in a similar situation may find it useful.
MyBaD is an MP3 player that aims to be a rewrite of Mike Oliphant's DigitalDJ. Details about songs are stored in a MySQL database, and playlists are generated by SQL queries through a simple interface. Compatibility with the database format of Grip will be maintained. The goals of the rewrite include a more flexible user interface, more modular code, and a faster release cycle.