I tend to agree with kkirsey: the built-in apps are completely sufficient for GTD. Learning how to use your tools can be more efficient than constantly searching for new tools. You may have other needs for a PDA that exceed the scope of list management, like richer multimedia features, but I would try using a plain vanilla setup before veturing into third-party apps -- even free ones. After you have some experience with the built-in apps, you'll have a better idea what they're lacking, if anything, and then you'll have a better focus in determining what aftermarket solution will fill the void. It's better in the early stages of setting up your GTD sytem to just get your lists, contacts and calendar entered. Establish the system first, then improve it.
David Allen's article on how he uses his Tunsten with no additional software is a good read: http://www.davidco.com/tips_tools/tip43.html