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, for all the file formats it supports.
Unlike some, Quod Libet will scale to libraries with tens of thousands of songs. It also supports most of the features you’d expect from a modern media player: Unicode support, advanced tag editing, Replay Gain, podcasts and Internet radio, album art support and all major audio formats.