MiniPlayer is a simple Mac program that was created to help iTunes and Spotify users control the music that's being played in their audio players. You can use this utility to play the previous or next song, pause, and resume playback, and check the artwork of the tune you're currently listening to.
The application features an intuitive and tiny main window which can be moved anywhere on your desktop. You can also choose to place the main screen on top of any application that's running on your computer.
Another great thing is that it uses a very small amount of CPU resources while running on your Mac and immediately updates the displayed info as soon as a new song starts playing in the iTunes or Spotify tools (Rdio app has been discontinued by its developer).
Unfortunately, the integrated search utility occasionally fails to provide you with the results you need. In my testing, I used the program to control the music that was being played in the iTunes app. MiniPlayer sometimes switched to iTunes' window once I pressed the Enter key.
The aforementioned aspect might prove to be a significant disadvantage to some users mostly because it doesn't provide you with an efficient way to switch to a certain song from your iTunes library. Hence, you need to access iTunes' main screen to search the song you wish to play.
With that being said, I think you can find better solutions for controlling the music that's being played in the iTunes or Spotify apps.