As a David Allen Getting Things Done (GTD) Trainer at my work, I have found many options for creating a trusted GTD System.

One solid consideration is to use a PDF or paper-based approach.

For $29, you can do this for 2025 using the GTD Organizer:

I have used this approach from time to time in the past when a paper system was more accessible. 

Editing PDFs makes this easy to use and platform-independent (Windows, Mac, Linux, etc).

1/1/2025 Update: Wired updated their 14 Best Planners, Notebooks, and Accessories for Organizing Your  Life.