Almost all of the mpris specification compliant media players are supported, including -

• Spotify
• VLC Media player
• Rhythmbox
• Audacious
• Music Player Daemon - MPD (support added by @M1ndo)

• Changing default player in config file
• View Help page by pressing h
• Read up the article on this application in Linux Magazine by Christoph Langner here (This was written for an older version, and a lot of new features have been added since then)