Songbird is indeed very good - it was my default player on Ubuntu for 2 years (from about 0.3 onwards) however of course on Linux development has recently been halted to focus on Win and Mac development... shame, but I hope that it does well.
For those who havent used it, it's the 'Firefox' of media players, both in terms of what it aspires to be figuratively, but also that it's an open, modular-type structure with some very effective add-ons.
It's also worth pointing out that for those, who like myself, actually get on with the layout of iTunes, Songbird nearly mirrors the look and feel. It's a very easy switch to make, and you can even skin the theme (like i said, like firefox) to make it look visually even more like iTunes.