There has been a whole Markdown renaissance going on on the interwebs over the past several months now. But while many Markdown-aware text editors such as TextMate have rolled their own previews for Markdown-rendered content from very early on, several tools are still lacking a useful preview capability. And if they have, they may not implement the fully-blown MultiMarkdown feature-set, which is the Markdown superset many Markdown interpreters support nowadays.
This is where Brett Terpstra's Marked comes in. It's a Mac app whose sole purpose is the rendering of markdown-formatted files that can sit anywhere on your hard drive. Preferably sitting next to your editor's window (such as iA Writer's), Marked will then watch your text file for changes and update its rendered output as soon as you save the file in your editor.
It has support for some advanced features such as custom CSS templates, but I haven't yet gotten it to a point where it'll preview a blog post in my full blog layout for example, to check line lengths for example. I'll play with this some more, though.
There's a great introductory video to Marked on Brett's site which is very much worth watching. Afterwards, Marked is available for $2.99 on the Mac App Store.
(Via Minimal Mac.)