Recs.
Updated
Wunderlist is a beautifully designed and simple to use to-do / task app you can use on multiple platforms including Windows, iOS and Mac OS X. It is free for everyone, you can unlock additional features with a Pro-plan.
Specs
Pros
Pro Background wallpaper
Wunderlist comes with 20 free wallpapers and 10 paid wallpapers for users to customize the background image in the Wunderlist app. In addition, advanced users can also use their own image as background image with few tweaks, which is absent in main Wunderlist app.
Pro Simple, useful task management features
Wunderlist features the standard ability to add tasks and organize them within lists, but one aspect in which it excels is the amount of detail you can add to each task. Rather than simple alerts, tasks incorporate a number of other options, including reminders, subtasks, notes, and recurring tasks. You can also star important tasks, which is simpler than the priority systems used in many other apps.
Cons
Con Sub-tasks hidden within the main task window
Wunderlist doesn't have true subtasks but rather a single task list with an entirely different user interface hidden within each task. Having separate interfaces for tasks and subtasks restricts flexibility in how one structures projects.