Music Player Daemon is a server-side music playing application which means it can be controlled over a network or locally using desktop apps. I've covered MPD before and I thought I'd expand on it since I have recently rediscovered how useful it really is. I like how you can control it from anywhere, the desktop, web, phone etc, and it has gapless playback and light resource usage. There's a massive amount of clients to control MPD, these are my favourites: (although some of these are essentially platform independent, I've focused on those that work on/with Linux)
My favourite desktop client is Ario, which runs on various UNIX/Linux desktops, and also Mac (using Fink) and Windows. It can find covers in the directory, amazon or Last.fm and can also 'scrobble' tracks to Last.fm too, though I usually use a server-based scrobbler such as mpdscrobble. It has a nice tray icon for controls and I find it the most usable of the desktop clients.
Client 175 is a browser based client that looks and acts like a desktop one. Make sure you have python-json installed and then download the latest Client175, then extract it (I used ~/home/carl/apps/client175) then you can run it from the command line with "python ~/apps/client175/server.py" (without the qoutes, obviously). To make it run as soon as my computer logs in, I added that command to Gnome's Startup Applications (System, Preferences), though there is a more complicated, proper way to do that here.
PhpMpReloaded is essentially a collection of basic web clients in one package and can be especially useful for browsing on basic low resource web browsers.
Maemo Music Player Client
MMPC is a version of the Gnome Music Player Client, with a focus on maximising screen usage and touchscreen usability. I have it running on my Nokia N800:
BitMPC is the only client that I cannot try myself at the moment since I don't have an Android device to try it on (donations welcome!) but I thought I'd include it anyway since I know many Android users who would find it useful. Also worth a try is PMix.