This was my first app on my own, and it's still got a place in my heart.
Long before Spotify had it, Synchstep was the first. Synchstep magically matched your music to your pace. It was pretty, simple, and a one-click dream for runners and walkers. Basically you just tapped start and off you go to enjoy your run without fiddling with options. Custom-built, hand-tweaked state-of-the-art step-detection that is still better than Spotify's. SynchStep had the largest collection of song tempos at the time, meaning more of your music worked instantly. At the time, lots of desktop apps would spend weeks calculating BPMs for music. Synchstep routed around this so you had no calculating tempos or waiting.
I spent forever on this app, and it was truly a labor of love.