I'm a Mac user and a long-time user of GTD. I tried Kinkless, and it is a good program, but found I really wanted something more simple.
So I just use iCal, which is free with your Mac and will backup and sync to almost anything.
To implement GTD I created a Calendar Group called GTD, and then created dummy calendars for each of the context groups (@Calls, @Office, etc.).
These dummy calendars are not used to enter appointments. I have a Home and Work Calendar for those and they are always checked. However, I do not enter any tasks in the Task Pane for them.
Instead, when I want to enter a note about a Call, I check the dummy calendar @Calls and a list of pending calls appears in the Task Pane of iCal.
If I want to see my list of errands, I uncheck the @Calls calendar and check the @Errands calendar and now the Task Pane shows my errands.
This works about as well as the Palm Desktop, and while you can't associate Projects with Tasks like you can in Kinkless, still it gets the job done and is very simple, direct and to the point.
Scot Giles