http://prokyon3.sourceforge.net/

The release candidate version has almost everything I want from a music metadata / organization application. Clean UI (despite being X-based), easy to mass edit tags, and cool bells and whistles like automatically importing album covers, band bios, album reviews, etc. And it stores all its data in MySQL, so adding on custom backend stuff looks pretty easy. The one big missing thing is customizable file renaming based on tag data (it only seems to have two hard-coded filename formats) but that's easily remedied with a bit of coding which I'll probably do and submit as a patch.

Anyway, if you're like me and are sick of relying on Windows apps manage your music, give this a shot.