Flex is a clean workout log book. You can add exercise sets and track you progress over time. As simple as that. I really like using it.

Shortcomings: It has no large exercise list with images or descriptions. Therefore, it is not really suited for beginners. It can be difficult/annoying to enter a new exercise because you can't simply add a set by clicking on a + somewhere on the screen. You always have to type some unique part of the name first. Once, you are used to this quirk, it's ok. There is no server, you can only use it on one mobile device. No IBAN for donations but at least monero.