I forked the excellent (open source!) Notational Velocity app, and hacked in a third pane that shows you the note you’re viewing as rendered by Markdown.

I just did a search while working on something else and was aghast at discovering I had not linked to this before. It really is a simple as the blockquote above. For those of us who write (or increasingly, live) in Markdown, being able to see your markup rendered in real time is a godsend. Seriously, go and get it. If you need to focus and not see two versions of what you are writing, simply slide the Markdown pane out of the way. It’s what I use and it’s really, really great.