A free app for Android, by Polycents.

This app is a handy interval timer for your HIIT interval training and workouts. You can track your work and rest periods during your workouts. You can also use this app for other types of interval training such as cycling, running, lifting weights, exercise, workout, stretching, boxing, MMA, or HIIT.

The app has an overall time as well as a high/low intensity interval and rest time between sets. You can set the overall time to a specific number of minutes or hours. You can also specify the type of training you want to do (foreground or background) as well as the type of workout (cardio or resistance).