OneTask ($4.99, iTunes) is a lightweight to-do tool that encourages you to focus on just one item on your list at a time. A slim task bar shows the to-do at hand no matter what window is active, and you can quickly skip a list item or work on it using keyboard shortcuts.
What's nice about OneTask is that it forces you to avoid multitasking. Although people typically feel quite busy when they're multitasking, when it comes to actually getting stuff done, singletaskers are actually more productive.
Check out OneTask in action below: