I read GTD several years back, and it revolutionized the way I manage my time. I have seen increases in my productivity that defy description.
I too was chomping at the bit to code some sort of simple app or plug-in to assist in the GTD way of, um, getting things done.
In the long run, I have found that pen & paper (for me, a nice moleskine notebook and a quality pilot razor point pen) work best. Interestingly enough, that's exactly what the author suggests (VERY loosely paraphrasing here - it's been a while): years of teaching this approach to time and task management have shown me that computer programs designed to ease the process don't do much in the long run but get in the way. He goes on to suggest keeping things simple with pen & paper.
I still tried a number of computer-based alternatives, even though the author explains quite clearly that he has historical evidence to suggest they don't work. I have to admit - if I ever meet the guy, he'd have every right to say "I told you so."
So I'll echo his advice to you: forget about writing (or finding) the GTD "killer app." Do it the old fashioned way, the way it's been proven to work. You won't be disappointed.