Actioneer is still around, making a password management application. That link mentions the inspiration from David Allen and GTD.
Yep, they had some cool technology for quickly capturing stuff in Palm, Outlook, and Lotus Notes, making it easy to empty your mind. Their software would parse a wide range of natural language to the appropriate lists, e.g. "call Bob" to your calls list with Bob's phone number, or "Mon 2p" to your calendar for next Monday at 2:00PM.