Just read this, over three years later --
As an exhaustive list, I guess it illustrates the range of choices that GTDers have for their trusted system. Kudos to the person who took the time to put it together.
Just a couple of things bug me about lists like this: 1) Even the best applications like these are often developed by just one person, and 2) just because someone calls their app "GTD" doesn't make it so.
People, you need to be careful -- don't put your entire project life into a GTD software that may not stand the test of time. If there's just one developer -- not a team -- and he/she gets bored, disinterested, too busy, married, divorced, whatever, you might find your GTD trusted system becomes stale beyond repair. And I hate to say this, because I'm a fan of open source, but open source software projects suffer a higher "abandon" rate, because they generally do not provide an income to the developer(s).
Also, as there is no "certifying" body for all the apps out there that claim to be GTD-centric, then you are left to trust that the developer actually knows as much or more about GTD as he/she does about writing code.
If there isn't a lot of buzz, reviews, testimonials, Google search hits, etc. about your prospective system, and if the app hasn't been updated in over a year, it's a very good indication that there is just one developer, and he/she isn't focused enough to be getting things done with the GTD app that they've written.