This post is for people who use and love Markdown and who have Macs. If you aren't in that niche market, move along (or join the club and begin to love Markdown, at least). I have constructed all kinds of system for various projects, including my favorite one for longer projects. It uses Tree.app and spits out a ready-to-go Markdown file (read about it here).
I recently downloaded Editorial for my iPad after a long wait caused by my satisfaction with my previous workflow (workflow sounds super important, doesn't it?). Before that I had happily been using Byword and Marked.
Byword has huge advantages because it is available on the Mac and iOS and syncs via iCloud without much effort from the user, and it's a fairly sophisticated Markdown editor. You can use keyboard shortcuts to preview your Markdown in HTML and then easily click to copy HTML and other formats right from the app.
Marked is a handy app that lets you preview your Markdown in real time. I used to have a skinny Marked window next to my Byword window, and that way I could see a prettified version of whatever I was writing. There is now Marked 2 (follow breadcrumbs from URL above), which has some fun shiny features, none of which I currently need, so I haven't bought it. I have not abandoned Marked, either, because I use Scrivener occasionally, and Marked does a fantastic job of rendering my Scrivenings prettily.
Anyway, I bought Editorial and fired it up. This will cause instant addiction, so I'm just saying, you should totally buy it, but be prepared for drooling and stuff. It has workflows and all kinds of geeky tools. Most handy to me is the insta-Markdown preview. On an iPad, you can't have side-by-side screens. I quickly reached a point where I didn't want to write anything on my Mac because there was no Editorial there.
So I searched and found Mou.app for OS X. As always, it depends on what you want from an app, but for my money (it's free, but donate!) Mou is the shiznit. You type your Markdown happily in the left pane, and it shows up next door in the right pane. If has fewer export options than other apps, but since it's Markdown, if I need something like an RTF or DOC file, I just copy the file somewhere else and export it. This is part of the beauty of Markdown.
Mou is short on features compared with some (maybe most) of the other Mac text editors out there, but I find myself using it almost all the time now. I find, however, that much of the geekery I like with other apps, including on the iPad with Editorial, can be had with PopClip for the Mac, which has nearly 100 free extensions that sort of approximate Editorial's workflows.
Anyway, I had never heard of Mou.app amongst the Mac-erati, so I felt I should give it a bit of a plug.