One of our favorite to-do lists/task manager apps for Android is Astrid. We love the deep feature set, the advanced tinkering and the easy to manage interface. Now it's on the iPhone. And even though the iPhone has a ton of apps like Astrid, Astrid is already one of the best.
To-do lists of the past were scribbled in notebooks, on sticky notes, or on whiteboards. Today, they live in our smatphones. Or at least they should.