I am glad you recommended the Getting Things Done book, it has been very helpful for me.

The book leaves it to the reader to decide the best medium to keep action lists (whether it be on paper, on palm, or on computer, ..). Which has left me trying to figure out what is the most effective for me.

I found the following free online tool for keeping track of action lists: http://www.toodledo.com/

I’ve been using this tool for about a week now, and rather like it so far. Once you create an account, and click on the learning link, you can see a link to the following article:
http://www.toodledo.com/info/gtd.php
(which, if nothing else, indicates that the folks at toodledo.com are aware of the “Getting Things Done” principles)

Within toodledo.com I like the you can specify tasks with a context, and then filter your action list based on those contexts. Some of my contexts include.. {In Car, With Computer, At Home, On Phone, At Work}. So before stepping out of the house I can filter on “In Car” and then print the list of things that I’d like to do while I am driving around.

I am curious which other tools people are finding useful for implementing the Gettings Things Done principles ?

You may find this site interesting: http://www.diyplanner.com/templates/official/classic a collection of document templates one may use to help put into practice some of the concepts you and the book you recommend utilize.