Recs.
Updated
Audacious, a descendant of XMMS is an opensource audio player that is low resource and has a minimal feature interface.
SpecsUpdate
Pros
Pro Lightweight
Audacious uses only 18.5 MB of RAM. It has a Winamp visualization style that's very minimized and convenient.
Pro The easiest player for multiple types of files - dts, aac multichannel, etc
Pro Streamlined player not weighed down by unnecessary options
Audacious is a classic music player at heart that has not felt the need to weigh itself down with an assortment of unnecessary options. Advanced functionality can be attained through plugins available from within the program.
Cons
Con Audacious is an audio player - not a Media Manager
Media managers are a different animal - for audio files, you have organisers (Musicbrainz Picard is my choice here) and advanced players (like Banshee, Clementine, Deadbeef).
For simply listening to music, Audacious is light, fast loading, handles playlists and works very well. For listening to music whilst also being able to delete and rate you're better off with something else - Banshee has my entire music collection in it's database for example.