Todoist is a task management and todo app with not only a great and easy to use interface but already supports pretty much any platform you might find yourself on. With support for iOS, Android, Windows Phone, Mac OS X, Windows, and many browsers, it's one of the best options for platform hoppers.
Aside from tons of cross-platform support, both native and through the browser, Todoist functions by allowing you to create both inbox tasks and tasks within project lists. Tapping on tasks will let you edit them, mark them as done, and postpone them. You can either choose to create a new Todoist account or use an existing Google login.
While there are tons of task management and todo apps out there, there aren't very many that give users the ability to jump between virtually any browser or desktop platform and have access to all their tasks. Todoist also offers a premium version for $29/year which gives you access to enhanced features including note and file attachments, better label systems, email tasks, and more.