Creative Writing on your Mac
The first great program for Mac is WriteRoom, it offers a none distracting full screen mode for writers (a bit like the old word processors of the 80's) but with smoother graphics. This program is great if you have the need to just sit down and start churning out text but many of us need to plan and focus on our storyboards before we get going.
This is where Scrivener comes in, it's a storyboard management program with a great intuitive interface. It will help you write a short story to a full on thesis. It helps you manage your writing project by providing a corkboard for you to drop images, text, PDF's and generally organise your research. It also comes with a full screen mode for focussed writing.
I should also mention that there is an alternative to Scrivener costing $35 and that is Avenir for $30, it does not have as many features as Scrivener but does offer a more common user interface that feels easier to use (think iPhoto's interface). The best part is that they all offer a 30 day trial download.