iPhone: To-do apps are everywhere, but finding one that's feature-packed, easy to use and based on the GTD system is tough. DashPlus is based on Patrick Rhone's Dash/Plus system, but fans of the GTD method might want to take a look at it as well.
As you'd expect, DashPlus is mostly about collecting a list of to-dos. How those are organised is what makes it interesting. You can organise your lists into a lot of different categories, tag them, review them regularly, move items to different actionable categories and more. The system is based on a bunch of symbols, where symbols show what needs to happen next. For example, a circle with a dash means an item has been moved to another list, where a left arrow means it has been delegated to someone else. You can also personalise that system in any way you like, create your own templates and move your tasks around easily.
It's a surprisingly powerful app considering the price, so if you're looking for a feature-packed to-do app, DashPlus is seriously worth a look.