Remember the Milk
With a name as interesting as that, you just have to read on. This is a popular productivity app that puts your tasks in a to-do checklist. It has its following of diehard fans. It lets you share your tasks with friends and leverages the power of checklist. The book The Checklist Manifesto really explains why checklists are so useful. Check it out for further reading, but essentially, it walks through top achievers like surgeons and architects who made crucial mistakes by not using a checklist.