MOC is a very convenient little application. It doesn't mess with any database junk (i.e. Rhythmbox), it just caches ID3 tags of files and plays them straight out off of your filesystem. It's got a clean, simple interface for browsing and setting up playlists. It also supports a myriad of codecs from the typical MP3/OGG/FLAC to weird ones like SPEEX and MODPLUG... in addition to ffmpeg supported audio codecs (like WMA) and that's not all. Also good for internet streaming radio from the command line.
The application starts a small server process so that you can close the ncurses interface and still listen to music (and eliminate all of those terminals floating around). But also provides you with simple command line options to get information on the currently playing song and control the player making it ideal for integration into your status bar or tying shortcuts to it.
All in all, I've been though a lot of audio players and they all just got in my way. MOC stays in the background, does what it's told and does it well. Perfect for those of us that avoid the mouse and or giant graphical applications trying to look flashy.
In the interest of full disclosure: I wrote a couple of patches late last year for MOC, but it's not my project. Besides, isn't that what you do with software you like, improve it?