Ethan J. A. Schoonover has made Kinkless GTD, an OmniOutliner Pro-based Getting Things Done system. It’s gorgeous, and it’s flexible, and it’s comprehensive, and it’s free. And he’s been extraordinarily generous in his response to comments and requests for features.
There are some issues yet to be worked out in the system, but I have every confidence that they will be.
(Not incidentally, I upgraded to OmniOutliner Pro yesterday, in part because of Kinkless GTD, but in part because I wrote last week’s lecture entirely in OO, and found the entire experience mind-expanding.)