Hello all. Thought I'd pass along news that someone's modified the third-party application OmniOutliner specifically for GTD. It's still a work in progress, but it's sharp and getting a lot of traction in a remarkably short time.
Here's an overview of the freeware mods:
And here's a link to a very Mac-centric GTD-oriented site that hails the KGTD modifications:
Finally, here's a link to the non-modified OmniOutliner software:
I should note that I'm not affiliated with any of these operations; just a Mac-using GTD fan who's always on the lookout for clean, attractive tools.