A GTD-like list, with reminders based in contexts (such time, position, talk/see/stay near somebody...).
It may use the Wishlist:Point_of_Interest_Framework.
When I arrive home (detected with GPS) and there are some "@home" things in the to-do list, then the system reminds me of them.
If I call Alice, then the system reminds me of the things I want to ask her before the phone makes the call.
If I meet Alice (bluetooth proximity sensor) the system reminds me of the things I want to ask her.
I want to talk to Peter about something and the system knows that I usually meet with Peter in the office.
I go to the grocery store and the system pulls up my grocery shopping list.
This can and should be rolled into Calendar without overly complicating it.
In a traditional GTD app, you have three columns: the task itself, the "context" and the "project".
A standard to-do list app, such as the one on the Palm Pilot, only has two columns: the task and the "category".
I propose that instead of either traditional app, we have an app with two columns: "task" and "tags". Tags are web 2.0 buzzword compliant and they provide a simple interface similar to that of a standard to-do list app. The only difference is that you can have more than one label attached to each task.
A more extended application called mebot is being developed with GTD in mind.
This may function like POIs (Point Of Interest) at other GPS devices. A deamon calculates the proximity of a list of POIs and current position and sends an call to the calender to show up a list tagged with a specific tag for this POI. If generic enough it will let other deamons and applications do the job as well, like Bluetooth senses your workstation and starts synchronisation. This Idea is further described in Wishlist:Point_of_Interest_Framework.