Both Spotify and Last.FM allow you to access a list of a user's recently played tracks. Multiple services exist for tracking exercise (Strava, Fitbit, Runkeeper, ...) and associated data (heartrate, pace, ...). I'd like to create an app that allows you to compare your exercise data with your recently played music over a large time period to determine whether the music you were listening to has any impact on your performance in the exercise.

This could be created as a secure web application accessible to anyone that utilises the APIs of the services to perform this analysis in real time.