Markdown is a Text-to-HTML Language/Utility by John Gruber which enables structured Plain Text to be translated to HTML. The Syntax is very easy, and will most likely already be the way you structure Text anyway.
So why do I got used to using Markdown Syntax in all my notes, and didn’t continue with simple plain text? The answer is quite simple, with Markdown I get a clear structured Document, with the advantages of HTML if I need it (links, images, headings, etc) without the need to actually write HTML. Plain Text always had the problem for me that it is absolutely not presentable, so every time I trough together a simple Text file on the side of a Project, I had to rewrite it if I actually wanted to use it for anything besides a reference for myself. With Markdown the I get a Document which can easily be sent to somebody else without being embarrassed by the look, and without additional work.
By using TextMate as my favorite Editor I finally got a quick way to take structured notes with easy preview, always ready.