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I too use Mou and it's a great editor. I've contacted the developer a few times (http://chenluois.com/ - bugs and feature requests) and s/he's been very responsive and usually emailed back in a few hours.

Markdown Pro (www.markdownpro.com - 6 dollars) is another great markdown editor for mac that has more features.

Also worth checking out http://www.iawriter.com/

It isn't free, but its design is phenomenal.

Edit: Just noticed Mou won't be free after beta. Will be interesting to see how the prices compare.

iCloud integration between iPad and mac app makes iawriter pretty awesome for me.

Texts - a Markdown editor for Mac & Windows - http://www.textseditor.com/

I like that it's a true wysiwyg editor in that you actually edit rich text instead of typing the markdown syntax.

A web version of that would be awesome to integrate with apps.

I almost wish I could have some kind of vim plugin that would render markdown. I tried finding this for MacVim (which could be capable) but alas... :)