Trello - Infinitely flexible. Incredibly easy to use. Great mobile apps. It's free. Trello keeps track of everything, from the big picture to the minute details.
nodo - Manage your daily tasks in the command line
Everlist Task Manager - Groceries, trips, errands, and daily todos managed simply. get your tasks under lovely control.
TaskPaper - Plain text to-do lists for Mac
Taskful - Deadlines, meet your match.
Simply.Digital - Goals & Tasks - Todo list to track your time & help set meaningful deadlines