Before they got bought by Adobe, Behance had a solid focus on GTD. Right after I left Poke / Mother, I got connected with Scott Belsky to help them make an iOS version of their online task management software.
My job was to help them understand how to adapt the web world into a mobile context (the iPhone 1 had SUCH A SMALL SCREEN!), and then take over a code-base from a previous developer that had created a technical graveyard of a stack. I had to transition and rewrite a ton of code in order to fix fundamental bugs. After that, I was charged with adding features such as resolving versioning conflicts between web and mobile, and implementing new features like the "Focus" section that was a beloved addition by the user base.
Here's the blurb Behance used to talk about the app, in case I missed anything:
Action Method Mobile is a free and powerful task management application, designed for maximum productivity. The app also syncs seamlessly with the award-winning online web application, Action Method Online. Action Method functions both as a personal and a collaborative task management application that allows users to organize tasks by project, due date, color, keyword, who delegated it, or the new "Focus List" feature.
The application is based on a methodology designed to make creative people more productive. Action Method lets you capture any and all Action Steps (tasks) and then automatically synchronize those Action Steps (whenever internet connectivity is available). Some users also chose to manage their projects online.