To make it even more versatile and flexible, their newsletter says:
Gtdagenda now syncs with Evernote, and you can attach Evernote notes to your Gtdagenda contexts and projects.
Evernote is the ultimate storage solution for your notes, your “External Brain”. Keep all your reference material in Evernote and link what you need to your Gtdagenda projects and contexts.
Here’s how to do it:
GTD Agenda helps you implement David Allen’s GTD Method, as well as the Zen to Done and 7 Habits techniques, and tailor it to fit your unique needs for school, home, and work.