Windows: Markdown is an awesome formatting language that lets you create rich documents using plain text syntax and MarkdownPad puts it to great use Windows.
The app is a dual-pane affair, with a preview panel on the right to show you exactly how your Markdown is formatting. You don't even need to click over to the panel to update it, since it updates live with every keystroke -- a very nice touch. Once you're done writing, you can export your document as plain text, as a markdown file, or as HTML if you're building web pages. If you aren't happy with how some elements render within the app, you can even add custom CSS to the app to change how they display.
The free version of MarkdownPad is incredibly powerful, but a $US14.95 upgrade to the pro version gets you a load of extra options, including a horizontal panel arrangement, PDF exporting and the ability to use a few different forks of Markdown if vanilla doesn't cut it for you. If you have any interest in Markdown, and you're a Windows user, this is definitely worth a download.
MarkdownPad (Free / $US14.95)