I admit it. I have failed more times using GTD then I care to admit. It was easier to quit smoking then get the process right. Though I think there is merit if you have the fortitude to stick to it.
I have tried many many todo apps. Many of them great - but did not fit my needs or the way my mind worked (or did not work).
I have tried:
Recently I came across a new application that seems to work amazingly well for me called Wunderlist by 6wunderkinder. It is very simple and beautiful. But don't confuse simplicity with lack of functionality. You can create context lists or different lists by function like work, personal or home. You can share lists - print lists and set do dates. My favortite feature of all is its cross platform synchronization. I use a Mac and a PC at work, a Mac at home alone with an iPad and iPhone. If you setup a free account on their web site you can synchronize all your tasks across multiple devices. A version for Android is also in the works to be released shortly. Opening up the application on any of the devices initiates the synchronization so you are always up to date.
If you live your life by a list this is a great application to make it a little easier.