Audacious is a media player application for Unix. It is fork of the Beep Media Player, which is in turn an updated version of XMMS which uses GTK2. It came about after BMP development mostly stagnated, and has progressed remarkably quickly. After some early pfumfering with such matters as plugins and i18n support, it seems to have finally matched most of XMMS' features as of version 1.3.1. While it can't use XMMS plugins directly, they require very little modification, and the majority of the popular plugins for XMMS have been modified for Audacious.
Linux is the primary development platform, but Audacious also works on FreeBSD, NetBSD, OpenBSD and Solaris. IRIX, AIX and HP-UX aren't officially supported, but will probably work without too much difficulty. Ports for Windows and Mac OS X don't exist yet, but shouldn't be particularly difficult either.