Mac: If you've decided to swear off multitasking and want to focus on one thing at a time, Tic Toc is a handy to-do manager for Mac that lives in the menubar and only shows you one item at a time.
The app can keep track of as many to-dos as you like, and adding more is easy, but you'll only ever see one to-do when you look up at the menubar, so you can single-task on that one thing until it's finished.
Tic Toc reminds us a lot of previously mentioned OneTask, which does something similar. OneTask doesn't live in the menubar, but it does keep track of time and lets you start, stop, pause and postpone items on your to-do list. Tic Toc on the other hand is simpler than that and just shows you what you should be working on. Click "Start" to begin timing your work, and stop when you're finished. To add a new task, just click the menubar icon. Plus, where OneTask lives on your desktop, Tic Toc is in the menubar, completely out of the way.
The app is definitely a departure from task-driven and list-based to-do list apps, but once you get started with it, Tic Toc is a remarkably powerful utility that can help you focus on specific items. It'll set you back $5.49 in the Mac App Store.