Wunderlist (Web, Desktop, Android, iOS, Kindle) is a to-do app that lets you create tasks, arrange them into lists, receive reminders about them, and share them with other people.
Pros:
Pretty
Fully featured
Easy to learn
Cons:
Are there any?
Todoist (Web, Desktop, Android, iOS) is a to-do list app that lets you create tasks, organize them into projects, sort them, and set productivity goals for yourself. Tasks can also have due-dates, and you can make them repeat at certain intervals.
Pros:
Many of the same 'pros' as Wunderlist
Earning karma can encourage you
Cons:
Many basic features require you to go premium