For Mac users who want an easier way to make lists and stay organized, TaskPaper is a simple to-do list that’s surprisingly adept.
Unlike today’s complex organizers, TaskPaper is just about you and your tasks.

You can even use it for your daily GTD plan! For me it's very important the simplicity and the easy to use, for my every day tasks and projects.

TaskPaper knows about four things: projects, tasks, notes, and tags. As you type, these items are auto-formatted so that your lists are easier to read.
It doesn’t force a particular system on you; it provides you with basic to-do list elements and then you use them as you see fit. Other programs can’t match the flexibility enabled by this clear text based interface.

TaskPaper isn't the right answer for everyone, also check out OmniFocus or Things. But you have to give TaskPaper a try!
It has a simple "text" user interface, but in some ways (Searching, AppleScript support, Themes, ...) it's more powerful and flexible then those bigger programs, and the last weeks I'm using it, I found that I'm really more productive with TaskPaper then I would be with a more complex tool.

Yes, Taskpaper rules!