While all GTDers will have their personal favorites, GTD is an approach that is not tool-specific. So while it's important to land on gear (paper or digital) that will stand up to the complexity of your work and personal life, it's more important that it clearly serves the purpose of reflecting the reminders and information in the most appropriate way for you. The tool won't decide what something means—you have to do that, and the GTD process is the key.
Check out our Common Tools page for the most common ones we've come across. We do not sell software ourselves, but the Common Tools page will direct you to source for those programs.