In addition to Viewfinder Preview I have also written a darkroom timer app for the iPhone. I wasn’t happy with any of the timer apps currently available so I wrote my own. It allows you to build your own development recipes with individual timers for each stage of the process. Because I don’t want to be messing about with my phone in the darkroom it is designed to be simple to use (touch anywhere on the screen to start/stop a timer) and easy to see the current state of the timers at a glance. You can find out more here.
The app is available on the App Store for free.