Apple iTunes
5
iTunes has been getting better and better over the years, but some of its best features are still a little hidden. My favorite tool is still the smart playlist feature, which allow you to create a playlist that automatically updates with new music according to any characteristics you can think of including song rating, date of last play, genre, you name it.
My favorite new feature has to be Genius, which, in most cases, creates an entirely fresh playlist of music I didn't know I liked or had forgotten about entirely, all from a single click of a button when I hear a song I really happen to enjoy at the moment. Since my taste in music fluctuates based upon my mood and what I'm doing, this is an amazing way to create a perfect playlist, instantly.
My only complaint about genuis is that it doesn't have enough data on occasion, and it works best as a tool to discover songs in your own catalog rather than for finding new music recommendations.
I also use iTunes rather prolifically for podcasts, which basically supersede the need for audio books in my life. The iTunes store makes it easy to discover new ones you might like via the sidebar recommendations they expose on each podcast page.
Last, and very much the icing on the cake without much practical purpose, their new audio visualizer is mesmerizing.