TaskPaper
I tried Omnifocus and Things, while they both have a lot of nice features, I was looking for something to help my focus and felt that too much clutter and information in the window was distracting. And as a geek I always want to fiddle with all the features and setups
I needed something that had the simplicity and focus of my paper system, but with the flexibility of adding items while in mail, or on the web, as well as moving items around easily within the app (from action lists to someday/maybe and back, for example), and a way to see both single action items and action items from projects at the same time based on context.
When I tried TaskPaper I knew I had found the right app for me, and I recommend giving it a try. It was very easy to set up. It has a Project List to give you an overview. It has tagging for contexts, multiple views of your tasks, notes (can be linked to an action item), links to outside files and apps. It is as easy as using a text editor, yet diligently links your contexts, projects, and notes into a useful GTD system.
When I am working from my lists, I like to turn off the toolbar and project headings, and just have a list of what needs to be done at that time and context.
TaskPaper has really helped my focus, I think about the actions, and not the software.