Ask HN: What mac apps are essential for GTD?

I just purchased a MacBook Pro, and I'm already in love with it. I want to get my life completely organized and synced well between my iPhone 4 and iPad.

What are the best apps I should download for getting things done (GTD)? What are apps that are essential for basic time, workload, and project management? Free apps are ideal, but good software is always worth paying for.