Workout Timer - Interval Timer is a free iPhone app designed for HIIT and Tabata training. The app offers unlimited timer creation with the option to classify timers into folders. Users have a high degree of flexibility in setting timers, supporting repetitions and unlimited time intervals. With about 100 types of sounds and voice readings available, users can customize their experience with favorite colors and reading texts. The app also features background notifications and a dark mode for user convenience.

Overall, Workout Timer - Interval Timer provides a comprehensive set of features for users engaging in high-intensity interval training, all accessible for free.