Mac: We all know multitasking is no good for you, but most to-do managers still overwhelm you with the sheer number of things you have to finish in a given time period. OneTask keeps track of all of them at once, but only shows you one at a time, helps you track how much time you spend on it and manage your following tasks easily.
Unlike other to-do management tools, OneTask takes up a tiny strip of space at the top of your Mac's screen, and uses keyboard commands to add tasks, search for to-dos you've entered, and to see and manage the to-dos you've entered. You can easily skip tasks this way as well, setting custom delays so you'll be reminded to start the to-do again when you're ready to work on it.
When it's time to start a task, just press the play button on it, and the clock will start. You go to work, and the app keeps track of what you're working on. When you're finished, you can tell OneTask that you're finished, or if you need more time and want to come back to it later.
OneTask has a bit of a learning curve, especially when you're used to other, more list-driven to-do managers. Once you start using it, however, you'll find it addictive. It's available in the Mac App Store for $5.49.