iOS: Glass Planner is a free hybrid calendar and to-do app that makes sure all of your to-dos show up right when you need them to, and you never miss an alert. You get an agenda-like view of all of your to-dos and appointments, and adding or arranging new items is quick and easy.
Like many combination calendar/to-do list apps, Glass Planner tries to show you what you should be working on in advance of it being due, instead of just alerting you when something is supposed to be finished. It also helps you make sure you're not trying to fit too much into one slot of time. Its agenda view isn't just empty space -- it shows you what you have on your plate at any moment, and makes adding new tasks easy.
All of your appointments, tasks and events have notes, and they're all in the same place for quick access -- no tabbing around menus or swiping through multiple screens. When you need to get work done, you can switch to the "Act" view, which is more of a traditional to-do list of just the items you have to get done. The app also has great support for customisable recurring to-dos, so even complicated events that occur on odd days and times are easy to schedule.
Glass Planner is a very GTD-friendly application, but you won't find a lot of GTD jargon and terminology in the app, so you don't have to be familiar with GTD to use it (although you'll appreciate the little touches if you are). Hit the link below if you want to give it a try. The app is free but includes a range of in-app purchases for added functionality.