Tools for email would be really nice, though, since you definitely can't do that on paper. And much of what he talks about seems very applicable to email, but I'm not sure how to actually do them in an email client. Email is like a microcosm of the entire GTD system.
Todo.sh (bash script to manage a text file) is quite nice. See screencast here - http://www.todotxt.com/library/todo.sh/
I have to agree with this comment. I've been using todo.sh as well and it works wonderfully. It's easily parsed, there is a community working on making it better. And there is even an initial port to Python as well.
I'm using a similar command line tool for managing my todo items: http://swapoff.org/DevTodo# Jan-Wijbrand Kolman