It's an incredibly powerful pattern, so much so that I'm surprised by how little you see it in the wild. We've also combined it with collaboration functionality so that cards can not only have multiple parents but can have different parents for different users. This aspect might be novel – I don't think I've seen it elsewhere, but obviously it's hard to know due to the sheer number of productivity tools that have existed (as evidenced by you pointing to Lotus Agenda, which I hadn't heard of til today, thanks!)
It's not for lack of demand. Mitch Kapor (Agenda) and Mozilla talent tried an OSS reboot, Chandler. It was so well funded and failed so spectacularly, that there's a book about it.
