I started playing with GTD, I checked out MonkeyGTD and d3, as well as others (one with a Ruby on Rails backend).
I like d3 for the simpleness, however there are some things I would look at changing.
Google Group Discussion
I want to get to a point where GTD is my main area for journal, tasks, project list, etc.
Some things I want to add:
checkbox for common tags such as estimated time,15min, 20min.
Also I want to customize the RSS options as there are pocketmod templates which can read rss.