foobar2000

I've been using Foobar2000 for a few years now, but unfortunately the lack of a Linux version led me to search for an alternative player that would work in Window & Linux. I can't find anything to match it. There are so many features of Foobar that I miss in other players: stable sorting, custom fields (codec, BPM, anything), replay gain, file operations, and so on. Admittedly Foobar isn't perfect out-of-the-box, but for anyone prepared to invest a bit of time setting up their ideal music player environment, Foobar2000 beats them all. (Plus, it isn't trying to be an iTunes clone!)

nothing beats the low memory footprint and versatility of MPD. And now it runs in Windows too!

Far and away the best audio player/music library organizer out there. I have yet to find an audio format that was not supported natively or by a simple "installation" (just drag' n' drop into the 'components' folder) of one of the many optional components (Like Ipod management and such) that the active and friendly community created.
And just in case you did not want to deal with the "steep learning curve," there are a number of quick setup options to choose from. Also the "layout editing mode" where you can adjust the User Interface in real-time is fantastic.
Although it is, sadly, not open-source, it does have a VERY expansive SDK (Software Development Kit).

Fast, light-weight, and downright elegant, I honestly can't sing it's praises enough.

DeadBeef is a foobar-clone for Linux Distros ,there is also another alternative Foobnix :D

Lightweight, it features all I need (keyboard shortcuts, intuitive playlist, never crashes.

Insanely customizable and with no doubt the best music player out there. To bad there's no linux version.

Extremely lightweight, feature-rich and runs great even on ancient hardware. Only problem I have with it is the lack of source code which for ethical and security reasons puts me off.