folks,
I have tried a simple and elegant tool which I like more than any other thicker tool that I have tried.

MonkeyGTD - awesome features - built on TiddlyWiki - works from my USB (also known as an extension of my body) - and preserves the original elegance (terminology, philosophy, etc) of GTD. Frankly I love it. check it out... It's way better than many/most of the ones listed here.
well - RTM does come close with it's gears based version - and I really love the way it parses dates intelligently - but that's more a task manager function than a GTD function. But if you ask me to pick , I will choose MonkeyGTD! (though, I need to admit, the learning curve is more for MonkeyGTD if you havenot used a tiddly before)