Bluemindo is a really simple yet powerful audio player in Python/PyGTK, using Gstreamer. It’s a free (as in freedom) software, released under GPLv3, only.
Bluemindo’s user interface consists of a main view of all your digitized album discs, which is searchable and filterable. The playback mode is gapless and support ReplayGain (it can also automatically choose to switch between Album and Track modes, depending on the kind of music that’s being played). Last but not least, it brings playlist (automatic and user-made), MPRIS/MediaPlayer2 support and lyrics/artwork downloading.