I can see that our product Agenda At Once is pointed out on this forum long time ago. In the meantime we have released updated version that implements some of shortcomings mentioned here.
As first version was out just few months after book is published (AFAIK), AAO was not originally bound to GTD methodology. Still, we made certain efforts to improve it in that way, while keeping previous features and philosophy for existing user base, of course.
Main and most feature-rich area of AAO is hierarchical to-do list management, but you can also enter fixed (scheduled) appointments, formatted notes and basic contact information. We tried to pack all these features into the unique user interface to make usage as transparent and simple as possible (see attachment). For details and trial download please visit product page:
I would be very interested to hear your opinions and ideas about further improvements and please accept my apology if this is not right place for this kind of questions.
Thank you in advance!