I don't use Markdown often, but when I do, I've been using the free MacDown program. But I always like to check out the alternatives, and MarkEdit has a lot going for it. For starters, it's half the size of MacDown (5.9 mb vs. 13.7 mb on my disk), the interface is clean (it really is like TextEdit!). The only thing that I didn't care for is that the program automatically installed a QuickLook Finder extension without asking. This feature may be a good thing, but adding yet another extension -- without asking first -- could be seen as intrusive or just plain rude. It can be removed or disabled through System Settings, but needing to do so seems counter-intuitive. One feature MacDown has that MarkEdit does not is a preview of the Markdown document as rendered in a browser; it really is a Markdown EDITOR.