You may have seen me quip about MarsEdit the other day on Twitter. Really, though, it is an excellent piece of software that should be given a go by anyone that writes a lot.
I'm spoiled by a rich-editor in Wordpress, that saves drafts automatically and even has multiple revisions, and so I'm sticking with Wordpress for now. But if I was going to use an application to write with - it'd be MarsEdit.
Oh, and the latest version supports Tumblr now. Which is excellent.