I was wondering if any of you use your iPhone to monitor cadence? I use my phone as a hrm/bike computer but a search in the App Store didn't yield any results using cadence.

I have never seen an app for just cadence but any cycling app should capture your cadence via a cadence sensor. Wahoo makes a good cadence sensor that is Bluretooth and compatible with an iPhone 4S or newer. You may need to change cycling apps if your current app does not allow connections with external sensors. If you don't have a cadence sensor and all you want to do is keep a specific cadence, without tracking the cadence, any of the metronome apps will work. Just set the tempo (number of beats) and pedal to each beat. It's crude but it works.