Astid's business model was "Free app, with paid for extra features". The free app was close to feature complete though: the extra features you could buy were fairly small & so buying the upgrades was really just making a donation to the devs. They may have expanded the paid services since I used the app though: I used it for a while and it was pretty good, but I found the UI a bit clunky and difficult to navigate once you wanted to do anything more complex than simple tick-box todo lists and I eventually gave up on it.