The Vorbital Player is a free MP3, Ogg Vorbis, Wavpack, AIFF, SND, and Wave file player. It uses OpenAL and is small and compact in order to take up as little screen space as possible (though you can, of course, resize the window if you would like a larger playlist). It will also display the album art for a track if it is present.
The functions of most of the buttons are obvious, but just in case you can't tell at first glance the folder-shaped button on the left allows you to add a file to the playlist and the minus symbol allows you to remove a file from the playlist. Double-clicking on a file in the playlist will cause it to start playing.