Enter Seconds, an app designed to either track and run HIIT workouts or any other timed workout periods (boxing and MMA are also fitness pastimes which usually depend on rigidly defined periods of endurance training). With a clean and simple interface, the app comes with a few preset interval-based workouts, or you can set your own intervals. You can even program music and playlists to go along with whatever you're doing.
It's definitely not an all-purpose app for working out or anything else -- even with HIIT, odds are that you're combining this type of training with other exercise in some way. Seconds isn't meant to track calories burned or just work as a standard stopwatch. It's specifically designed to work as an interval timer. For that specific purpose, it serves the role well. Rather than fumbling around with a stopwatch and doing the math as you sprint around a track, Seconds makes it really easy to just let your iPhone handle the timing, so you can focus on getting the most of your HIIT workout. It's available for an introductory price of $1.99 in the App Store right now.