TickTick is a free to-do app accessible over a range of devices at the same time. As well as a conventional tick list, it also contains a calendar list which can be integrated with your device's own calendar. TickTick is also available across platforms through an app on mobile devices or a web platform via Chrome extension. Tasks can be shared with others through emails or links and recurring tasks can be created and customised. A pro version is available at a fee.