Furiousfive is website app delivering you HIIT workouts. No ads, no registration. They have app but only for apple(iOS). Have some trackers.
They are using a few trackers like google-analytics and some possible “ads”, though I cannot fully confirm it about the ads. Even without an adblocker I could not see ads, but with the adblocker on I can see some urls being blocked. Also, they are offering an app version of the website only available on the Apple Store that requires a trade-based operating system to access. However one can use the app online.