Date Reminder - Date Reminder reminds you of recurring or nonrecurring events, like birthdays, bills to pay...
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.
todoMini - Your web based TODO list.
Reminduck - A simple reminder app made for elementaryOS with Vala and GTK. - matfantinel/reminduck
Reminder Commander - Reminder manager that reminds you about important events in advance.
Remember The Milk - Remember The Milk is a task and time management application for mobile devices.