Notepad++ is still useful in a pinch, and it's free for the downloading. If you have a Windows machine or VM, you should have Notepad++ installed, ready when you need it.
Once upon a time, TextMate ($55, OS X 10.7 and higher) was all the rage among the cool kids who wrote Ruby on Rails on their MacBooks while sitting at tables in college cafés. TextMate has since become less prominent as it fell into neglect and Sublime Text gained popularity, but with version 2.0 the product could be ready to make a comeback.
Markdown support is provided in one of the in-package bundles. It includes a document preview function, a Markdown cheat sheet, and lots of shortcuts for generating Markdown mark-up.
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