wger - FLOSS workout, fitness and weight manager/tracker written with Django

Intervally - Interval Timer - Sport & Health

Strong.app - Strenght training logger.

HIIT Interval Buddy - Let Interval Buddy help you with your high intensity interval training - a popular method of high-impact cardio fitness training that reduces time spent in the gym and makes cardio training less of a bore.

Strongify - Strongify is the easiest workout tracker app. Enter the number of reps and the weight you lifted in just a few taps. Create any exercise that you want and track your progress over time.

HIIT (Watch) - A very simple interval timer engineered for Apple Watch