It seems I'm the only one to use... TAGS! Wait, I'm not crazy. This is due to two main needs:
- One task may belong to more than one project. This is common, for me.
- Handling contacts.
So I do not use folders or goals (a single task would belong to a single project). I find tags to be very smart for adding/filtering and provide multiple binding.
If you turn up your nose for using the same column for both contacts and projects, you can prefix @ for contacts and # for projects. But for me, mixing contacts and projects (without prefix symbols) it's not a real problem and I find it very convenient.
Goals have a different and specific destination, so I avoid it for projects. I also avoid tasks/subtasks because I see them to target action/task difference.
I see two remaining options: folders and tags.
The key factor for the decision probably is the Projects number. Folders are ok when a small number of projects is involved. In my case projects number tend to explode, so I use... Tags!
Tags have also the great benefit to allow an action to belong to two or more projects.