For years, I've struggled with finding a decent set of tools to practice Getting Things Done. I started with the Hipster PDA and moved on to some notepads and Zen To Done. I've tried just about every permutation of Web-based application. Heck, I liked GTD-PHP that I bought the domain and host the project for free.
But none of them have worked for me. The Hipster PDA was good but it suffers from all of the problems of paper-based systems: there's no indexing or searching and you always have to carry around a pen. The notebooks were even more inconvenient; the desktop applications didn't help if I wasn't carrying my MacBook which I almost never do; and the Web applications had monthly fees and required a computer with Internet access to function. So I bided my time and kept on the lookout for the Holy Grail.
I can confidently tell you that I have found it! It's OmniFocus by OmniGroup. It's Mac-only and $79.95 but it suits me perfectly. Its power stems from the fact that the desktop application at home can sync with the desktop application at work which can sync with the iPhone application in my pocket. $79.95 (plus $19.99 for the iPhone version) is certainly expensive, but I found it invaluable after using it fully for its 14-day trial.
It has a bunch of nice touches: parallel or sequential task lists, quick entry that really is, the focus modes. It's a competent implementation of GTD—the incidentals of the UI aren't important. But the syncing is worth every penny of the cost. When you're away from a computer, you have access to your projects and contexts. Check something off and it's synced to your home and work computers. It just works—you never have the problem of managing database files and shuttling them between the computers in your life.
And that makes getting things done the focus rather than maintaing your system.