Quod Libet is a GTK+-based audio player written in Python, using the Mutagen tagging library. It's designed around the idea that you know how to organize your music better than we do. It lets you make playlists based on regular expressions (don't worry, regular searches work too). It lets you display and edit any tags you want in the file. And it lets you do this for all the file formats it supports.
Quod Libet easily scales to libraries of thousands (or even tens of thousands) of songs. It also supports most of the features you expect from a modern media player, like Unicode support, tag editing, Replay Gain, podcasts & internet radio, and all major audio formats.
Requires GTK+ to run.