Workout Music or Fitness Apps?

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Not sure if I'm just late to the rodeo but yesterday I saw on Twitter someone mention the fitness apps they use and one of them was an app called "FitRadio". Upon checking it out I figured out I really liked it and was surprised I had never heard of it before so it begs the question - what smart phone app(s) other than MFP do you use for working out or music or just that you highly recommend? I want to make sure I'm not missing out on any other great apps that I just haven't heard of before.