I am a passionate GTD fan since I read David's book about 2 years ago. It has worked great for me so far, but I never found the tool that just "felt" right. There were always to many bells and whistles or the sync across Desktop and Mobile Devices wasn't great.
I recently came up with a rather simplistic GTD solution that works very well for me so far. It helps me to focus on the GTD methodology itself rather playing around with the software. If you are interested, I put it down in a short guide, and I hope it might also be helpful to other people.
This approach is entirely free and would cost you nothing. You can even use it without simplenote but with just the tools your favorite operating system has to over, what makes it highly portable and future-proof.